Properties and Oral Structures - NISTCONTENTS Page ABSTRACT i FY88SIGNIFICANTACCOMPLISHMENTS ii...

NISTIR 89-4048

Properties and Interactions

of Oral Structures andRestorative Materials

J.A. Tesk, J. M. Antonucci, G. M. Brauer, W. G. de Rijk, J. E. McKinneyJ. W. Stansbury, S. Venz, C. H. Lee, A. Sugawara, K. Asaoka

U.S. DEPARTMENT OF COMMERCENational Institute of Standards and Technology

(Formerly National Bureau of Standards)

Institute for Materials Science and Engineering

Polymers Division

Dental and Medical Materials

Gaithersburg, MD 20899

Annual Report for Period

October 1 , 1 987 to September 30, 1 988

Issued May 1989

Interagency Agreement

Y01-DE30001

Prepared for:

National Institute of Dental Research

Bethesda, MD 20892

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NISTIR 89-4048

Properties and Interactions

of Oral Structures andRestorative Materials

J.A. Tesk, J. M. Antonucci, G. M. Brauer, W. G. de Rijk, J. E. McKinney

J. W. Stansbury, S. Venz, C. H. Lee, A. Sugawara, K. Asaoka

U.S. DEPARTMENT OF COMMERCENational Institute of Standards and Technology

(Formerly National Bureau of Standards)

Institute for Materials Science and Engineering

Polymers Division

Dental and Medical Materials

Gaithersburg, MD 20899

Annual Report for Period

October 1 , 1987 to September 30, 1988

Issued May 1989

Interagency Agreement

Y01-DE30001

National Bureau of Standards became the

National Institute of Standards and Technology

on August 23, 1988, when the Omnibus Trade and

Competitiveness Act was signed. NIST retains

all NBS functions. Its new programs will encourage

improved use of technology by U.S. industry.

Prepared for:

National Institute of Dental Research

Bethesda, MD 20892

U.S. DEPARTMENT OF COMMERCERobert Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDSAND TECHNOLOGYRaymond G. Kammer, Acting Director

CONTENTS

Page

ABSTRACT i

FY 88 SIGNIFICANT ACCOMPLISHMENTS ii

STAFF BIOGRAPHICAL SKETCH iii

INTRODUCTION iv

PART I - COMPOSITES, CEMENTS, AND ADHESION (Brauer, Stansbury,Antonucci, Venz

,Sugawara, and Lee)

Section A through D - Brauer

A. High-strength Eugenoi-free Adhesive Cementsand Restorations • 1

References. . 3

B. Changes in Esthetic Properties of DentalResins on Aging 5

C. Development of Radiopaque Copoiymeric DentureResins 7

References 9

D. Study of Adhesion and Adhesion Promoting Agentsto Dentin 10

References 24

Publications 26

Manuscripts Under Review or Accepted for Publication . . . .26

Contributed Talks 27

Section E - Stansbury

E. Monomers Which Polymerize with Expansion 28

Invited Talks 29

Section F - Antonucci

F. Improvement of Dental Composites, Sealant Cementand Adhesive Materials 31

I. Improvement of Dental Resin Systems forComposites and Sealants 32

II. Improvement of Dental Cements 43

III. Development of Improved Interfacial BondingSystems and Fillers for Composites and Cements . 47

IV. Bonding of Low Surface Energy Resin Systemsto Tooth Structure 59

References 61

Invited Talks 66

Publications 67

Manuscripts Accepted for Publication 67

Manuscripts Approved by WERB 67

Manuscripts Under Review by WERB 67


PART II - WEAR RESISTANCE AND DURABILITY ASSESSMENT OFOF DENTAL COMPOSITE RESTORATIVE MATERIALS(McKinney, Antonucci, de Rijk and Venz)

A. Wear and Durability Assessments of CompositeRestoratives 70

B. Assessment of Wear of Human Enamel AgainstRestorative Counterfaces 76

C. Glass -Transition Temperature (Tg

)

of Matrix Polymers. .77

D. Wear Instrumentation 78

References 79

Publications 81

Manuscripts Submitted to WERB 81


PART III - DENTAL ALLOYS, CERAMICS, AND METROLOGY(Tesk, de Rijk and Asaoka)

A. Porcelain-alloy Compatibility (Thermo-mechanical Stress) 83

B. Porcelain-alloy Compatibility 84

C. Castability (Filling of a Mold with CastDental Alloy) 85

D. Castability (Accuracy of Fit of Dental Castings) . . . .87

E. Solderability 87

F. Metrology and Analysis: Measurements forCharacterization of Dental Materials 87

G. Metrology and Analysis: Occlusal Force Indicator. ... 87

H. Metrology and Clinical Performance 88

I. Metrology and Analysis-Stress in Dental CompositesBonded to Teeth. 89

J. Metrology Sterilization of Dental Implements 89

References. 90

Publications 91

Manuscripts Under Review or Accepted for Publication. ... 92

Invited Talks 92


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ABSTRACT

The research program described herein is designed to achieve a number ofobjectives leading to improved dental restorative materials, techniques andapplications of dental materials science for improved dental health care in

general. Some of the research in dental composites is directed towarddeveloping generic polymer science potentially useful for compositeapplications, e.g., durable resin matrices and stronger more durable couplingbetween fillers and resins. Improved reinforcement is sought by defining the

type, and percentages of fillers which will result in improved performance of

composites. Methods for reducing polymerization shrinkage and attendant stressand marginal leakage are also explored. Cements are investigated and basicformulations developed for lower solubility, higher biocompatibility

,higher

strength, greater toughness and adhesion to various substrates including enameland dentin. Analysis techniques include IR spectroscopy, chromatography, x-rayanalysis, mechanical testing, and dilatometry. Another major effort is

directed at elucidating the fundamentals involved in wear and degradation of

dental composites and restoratives. Wear and hardness measurement techniquesare pursued as well as by identification of the origins and sources of flawsleading to failure. Weibull statistical analysis is expected to provide usefulinformation for this task. In this regard an objective is to investigateimproved correlations between clinical results of wear and failure ofcomposites with laboratory test data via time- to -failure analysis. Metrologyand analysis constitutes the underlying theme of investigations into porcelain-metal systems, casting of dental alloys and the expansion of dental castinginvestments

.

"The activity covered by this agreement consists of work which requires the

definition of measurement methods, materials property data, and standards ofbasic scientific and engineering units and the application of primarystandards to insure equity and comparability in U.S. commerce, internationaltrade, and technical activities. As such it complies with 0MB Circular A-

76, revised under paragraph 5f (Activities classified as Governmentresponsibilities or are intimately related to the public interest)."

i

FY 88 SIGNIFICANT ACCOMPLISHMENTS

o A new class of difunctionai monomers which can cyciize during polymerizationhas been developed for use in dental composite formulations. The resultinghomopoiymers exhibit high elastic moduli and extremely low levels ofresidual unsaturation.

o A spiro orthocarbonate monomer designed to maximize double ring openingunder near ambient polymerization conditions was prepared. Initial resultsindicate that the free radical polymerization occurs exclusively by the ringopening mechanism which leads to volumetric expansion.

o A series of substituted o-methylene lactone monomers was synthesized andpolymerized. The degree of solvent resistance in the resulting non-crosslinked homopoiymers was related to the various substituents. Ingeneral, solvent resistance was excellent.

o Several types of resin-modified glass- ionomer cements were developed whichyield tough, more hydrolytically resistant hybrid cement composites.

o For at least one composite resin it was demonstrated that after chemicaldegradation the diametral tensile strength values are described by a timedependent mixed Weibull distribution.

o A prediction on the service life of resin bonded dental restorations wasmade by treating clinical data as censored data in a Weibull distribution offailure times.

o With a microwave sustained gas plasma, spores of Bacillus subtilus wereinactivated after an exposure time of 30 seconds.

o The stresses developed in a porcelain-metal slab were calculated by computersimulation.

o Ultrasonic measurements of two model dental composites showed that Poisson'sratio is not affected by the use of filler-resin coupling agents. This canbe interpreted as evidence of residual radial compressive stresses aroundthe filler particles.

o Completion of iji vitro wear testing and environmental resistance of a

fluorinated flexible-resin composite. This material had higherenvironmental resistance to intraoral fluids, but lower wear resistance thanconventional composites.

o Completion of iri vitro wear testing and environmental resistance of a hybridcement-composite. This material, comprising a glass - ionomer cement andpolymerized water-soluble monomer, displayed good wear resistance andcomplete absence of brittle fracture during wear customary for conventionalglass - ionomer cements.

o Effect of topical fluoride-gel treatments on a commercial- radiopaquecomposite revealed a significant weight loss from successive treatments intopical fluoride gels, but no significant change in wear resistance.

ii

o In vitro wear tests of h\:man enamel against a commercial castable -ceramicrestorative revealed enhanced wear of enamel when a staining glaze wasapplied (as recommended by the manufacture) to the ceramic.

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H

DENTAL AND MEDICAL MATERIALS GROUPJ.A. Tesk, Group Leader

NBS PersonnelJ.M. AntonucciM.Y. ChiangW. G. deRijkS.F. HarlowJ . E . McKinneyJ.W. Stansbury

Research AssociatesPaffenbarger Res. Ctr. NIDR

R.L. Bowen, Director E.D. EanesW.E. Brown, Dir. Em. B.O. FowlerR. L. Blosser A.W. HailerC. M. CareyM.W. ChalkleyL

.

C . ChowM. L. ConnerF. EichmillerN. EidelmanA. A. GiuseppettiT.M. GregoryA . D . Johns tonD. KirkpatrickA. LyW.A. MarjenhoffM , MathewD. N. MisraE. F. PrestemonN

.

W . RuppC.T. SchreiberB. SieckS . TakagiB.B. TomazicM . S

.

TungG. L. VogelR.M. Waterstrat

IndustryS . Venz

,

DentsplyR. Muller,Astron Corp.

Guest Scientists

K. Asaoka, Tokushima Univ.G. Brauer, Retired NISTG. Buchness, Univ. of MarylandT. Chen, U.S. Food & Drug Adm.A. Chohayeb, Howard Univ.E. Cobb, Georgetown Univ.R. Engler, U.S. NavyL. Forsythe, U.S. NavyY. Fukase, M.

M. Gilberts, U.S. NavyW. Grant, A.D.A.B. Heywood, NIDRH. lizuka, Nihon Univ.

Consultants

C. Lee, W. China U. of Medical Sci.

W. Levine, Georgetown, Uni.M. Markovic, Hebrew Univ.J. Meyer, NIDRR. Penn, Retired NISTG. Santulli, U.S. NavyY. Sato,A. Sugawara, Nihon Univ.V. Thompson, Univ. of MarylandM. Waterman, U.S. Navy

R. Natarjan, U. of IllinoisP. Sung, U.S. Food & Drug Adm.G. Widera, U. of Illinois

iii

PROPERTIES AND INTERACTIONS OF ORALSTRUCTURES AND RESTORATIVE MATERIALS

INTRODUCTION

The following pages contain reports on work involved with the development of

basic generic science and engineering which is expected to be useful in the

development or control of dental materials used for restorative or treatmentpurposes. Some of the developments involve investigations into new dentalresin formulations (Part I) which might improve the performance of dentalcomposites. Cements and adhesion to filler particles or tooth structure arealso addressed in this part.

Part II deals with examination of the basic parameters affecting the wearand durability of materials with particular emphasis on dental composites. Theresultant information is used to help guide developments in Part I.

Part III is concerned with dental casting alloys, and the strength ofveneered dental systems (in particular, porcelain fused- to-metal) metrology,diagnostics and related topics. Factors affecting the castability of alloysand how to measure and define aspects of castability are addressed.Mathematical methods are employed to reveal effects of individual elements as

well as other parameters such as investment variations. The strength ofveneered systems is the characteristic receiving the most attention for theporcelain-fused- to-metal studies. Special emphasis is being placed onmeasurement techniques and flaw analysis. Weibull statistics is employed foranalyses of the strengths of dental systems, time to failure etc. A specialeffort has also been mounted to explore the use of plasma's for sterilizationof dental instruments. (Measurements of spore populations etc. will beconducted with cooperation of the U.S. Navy).

iv

I. COMPOSITES, CEMENTS AND ADHESION

A. High-strength Eueenol-free Adhesive Cements and Restorations

Overview

Cements are used in over 50 percent of ail dental restorations.Zinc oxide-eugenol type cements, because of the excellentbiocompatibility of the hardened material, are employed for suchdiverse applications as cementing media for crowns and bridges

,

sedative and insulating bases, temporary restorations, pulpcapping agents, root canal sealers, soft tissue packs and as

impression pastes.

Cements in current use are far from ideal. Their relatively poormechanical properties, high solubility and lack of resistance to

wear and disintegration deter their more extensive use, especiallyfor cementation for permanent prostheses or for their use as

intermediate restoratives.

Non-eugenol containing cements based on vanillate esters, o-

ethoxybenzoic acid (EBA) and zinc oxide, have been developed in

this laboratory [1-7]^. These cements have the followingadvantages compared to the presently used Zinc Oxide-Eugenol (ZOE)

or EBA cements: (1) excellent strength, (2) much lower solubilitythan zinc oxide-eugenol cements, (3) do not inhibit free radicalpolymerization and can be used in conjunction with compositefilling materials to which they adhere, and (4) are compatiblewith acrylic monomers and can be formulated in conjunction withthem, and (5) adhere strongly, even on prolonged water exposure,to non-precious metals, porcelain and composites. The cementsexceed greatly requirements of ANSI/ADA Specification No. 30 forType II, III, and IV restoratives.

Objective

The initial objective of this study was to synthesize and evaluatethese cements for various dental applications. To achieve thisobjective the following tasks were undertaken: (1) synthesis andevaluation of divanillates and polymerizable vanillates such as

methacryloxyethyl vanillate and addition of these compounds to

hexyl vanillate -ethoxybenzoic acid (HV-EBA) cements to improvemechanical properties; (2) quantitative measurements of the

adhesive properties of HV-EBA cements; (3) formulation ofintermediate restorative materials (IRM) incorporating monomersand reinforcing fillers with cement ingredients and determinationof their mechanical properties; (4) synthesis of cementscontaining the potentially caries -reducing syringic esters and

^Figures in brackets designate references included at the end of

this text

1

evaluation of properties of the resulting cements; (5)

modification of the cements by addition of small concentrations ofadditives such as acids, metals or fluorides to improve theirproperties and (6) furnish assistance and guidance to conductinvestigations of the biocompatibiiity and toxicity of the cementand its ingredients and collaborate in studies of the pulpirritation and in clinical studies at various dental researchcenters

.

Accomplishments

Objectives 1-5 have been completed. Results of these studieshave been summarized in References 1-16 and in the previousreports. These investigations have led to a number of cements andintermediate restoratives of potentially great usefulness forclinical dentistry.

Present objectives will be directed to (1) further improvements ofthe vaniliate or syringate cements by addition of modifyingagents, (2) increase the scope of the usefulness of thesematerials for clinical dentistry, and (3) complete studies of thebiocompatibiiity of syringate cements and cement-compositescontaining acrylic monomers.

PROGRESS REPORT

Phase I. Further Improvement of Properties of Vaniliate andRelated Cements

Because of the emphasis placed on other portions of researchconducted under this interagency agreement, no studies werecarried out on this phase during the year.

Phase II. New Applications of These Materials

No studies were conducted on phase II during the year due to

emphasis on other portions of the research. One reviewstressing the commercial suitability of these cements for

diverse applications has been prepared for publication.Another review summarizing the properties of these cementsand intermediate restoratives has been published [18].

Phase III. Study of the Biocompatibiiity and Clinical Usefulnessof Vaniliate and Syringate Base Restoratives

(1) Biocompatibiiity

Past Accomplishments

Vaniliate and syringate cements prepared in this laboratoryhave been subjected to the biological tests suggested in the

2

ANSI/ADA specification No. 41. They pass all these tests.

Their pulp irritation is similar to ZOE cements. Summariesof these studies are given in the previous annual reports andreferences 9-14 and 16-17. These cements have been madeavailable to interested parties for selected clinicalstudies

.

Progress

(a) Biological Evaluation of Zinc Hexyl Vanillate Cements

The cellular and tissue response studies to hexyl vanillate(HV)

,ZOE and zinc phosphate (ZP) cements were continued by

Dr. J. C. Keller and coworkers at the Medical University ofSouth Carolina. The cellular and connective tissue reactionsto HV approximated those of ZOE and ZP cements anddemonstrated the acceptable biological performance of HVcement. The results have been written up. The manuscripthas been accepted for publication in "Dental Materials '*

.

(b) Biological Evaluation of Vanillate Intermediate Restoratives.

Studies at the University of Texas at San Antonio andUniversity of Tennessee are continuing to determine the

biological characteristics of formulations- synthesized in

this laboratory containing dicyclopentenyloxyethyimethacrylate or cyclohexyl methacrylate and silanized glasswhich have much improved properties compared to commercialintermediate restoratives.

(2) Assistance in Clinical Evaluation

The use of these cements in clinical studies is encouraged.Materials have been synthesized for Dr. Baez at the DentalSchool, University of Texas at San Antonio, who subject to

funding, is interested in such a project. A clinical studyof the usefulness of these cements as restorative bases is

being outlined by Dr. Parker of the Navy Dental SchoolBethe sda

.

The vanillate cement have been used in a limited . clinicalstudy at the Louisiana School of Dentistry. No adverseeffect were noticed in their use.

References

[1] Brauer, G.M., Argentar, H. and Stansbury, J.W. CementitiousDental Compositions Which Do Not Inhibit Polymerization, U.S.

Patent 4,362,510, December 7, 1982.

3

[2] Brauer, G.M. and Stansbury, J.W. Development of High-Strength,Acrylic Resin Compatible Adhesive Cements, J. Dent. Res. ^ 366-

370 (1983).

[3] Stansbury, J.W. and Brauer, G.M. Divanillates and PolymerizableVanillates as Ingredients of Dental Cements, J. Biomed. Met. Res.

19 715-725 (1985).

[4] Brauer, G.M. and Stansbury, J.W. Intermediate Restoratives from n-

Hexyl Vanillate-EBA-ZnO-Glass Composites. J. Dent. Res. ^ 1315-

1320 (1984).

[5] Brauer, G.M. and Stansbury, J.W. Cements Containing Syringic AcidEsters -o-Ethoxybenzoic Acid and Zinc Oxide. J. Dent. Res. ^ 137-

140 (1984).

[6] Brauer, G.M. and Stansbury, J.W. Biocompatible CementitiousCompositions. U.S. Patent 4,486,179 Dec. 4, 1984.

[7] Brauer, G.M., Stansbury, J.W. and Flowers, D. Modification ofCements Containing Vaniliate or Syringate Esters. Dent. Mat. 2

21-27 (1986).

[8] Stansbury, J.W. and Brauer, G.M. Properties of Vaniliate andSyringate Cements Containing Various Fluorides. lADR Prog, andAbst. i4. No. 314 (1985).

[9] Siew, C.,

Gruninger, S.E., Hsu, R.,

Brauer, G.M. and Tesk, J.A.

Biological Safety Evaluation of a Newly Developed High StrengthAcrylic Resin Compatible Adhesive Cement. lADR Prog, and Abst. ^No. 1473 (1985).

[10] Siew, C., Gruninger, S.E., Brauer, G.M. and Tesk, J.A. BiologicalSafety Evaluation of Syringic Acid Ester Cements, lADR Prog, andAbst. 65 No. 1337 (1986)

.

[11] Miller, E.G.,

Washington, V.H.,

Zimmerman, E.R. and Bowles, W.H.

in vitro Studies on the Matagenicity of Dental Materials. lADRProg. Abst. ^ No. 1278 (1984).

[12] Kafrawy, A.H. and Phillips, R.W. Pulp Reactions to HexylVaniliate -Ethoxybenzoic Acid Cement. lADR Prog, and Abst. ^ No.

2 (1984).

[13] Rubinoff, C.H. and Greener, E.H. Physical Properties of an

Experimental Dressing Material. Dent. Mat. 1. 3-6 (1986).

[14] Glickman, G.N.,Goldberg, H.

,Loevy, H.

,Gross, R.L. and Greener,

E.H. Sealing and Biocompatibility of an Experimental EndodonticSealer. lADR Prog, and Abst. ^ No. 77 (1985).

4

[15] Stansbury, J.W. and Brauer, G.M. Bonding of Vaniiiate andSyringate Cements to Various Substrates. lADR Prog, and Abst. 64No. 393 (1984).

[16] Keller, J.C., Hammond B. and Kowalyk, K. Biological Performanceof Zinc Hexyl Vaniiiate (ZHV) Cements. lADR Prog, and Abst. ^No. 1343 (1986).

[17] Kafrawy, A.H. Pulp Reactive to Hexyl Vaniiiate IntermediateCements. J. Dent. Res. Abstract of Papers, £6, 176 (No. 553),(1987)

.

[18] Brauer, G.M. Vaniiiate and Syringate Cements. Trends andTechniques in the contemporary dental laboratory 5 (No. 1), 6

(1988)

.

B . Changes in Esthetic Properties of Dental Resins on Aging

Overview

Although the color stability of the presently employed dentalresins is quite satisfactory, some discoloration of thesematerials can often be observed clinically. Under oralconditions, these restorations are exposed to the combined effectsof light, moisture, stains and mechanical wear resulting invisibly detectable, and aesthetically undesirable color changes.Many of these changes are the results of photochemical reactionsof ingredients in the composite caused by exposure of therestoration to various energy sources. Since clinical studies to

determine color stability of restoratives are time-consuming,accelerated aging tests have been suggested to correlatelaboratory findings and clinical performance. Most of these testsare based on short-term (24 hour) exposure of the materials to

light sources or heat, often in an aqueous environment .

Objective

The objective of this study was to (1) investigate the colorchanges of a wide variety of dental resins resulting from theexposure to different radiation sources or to thermal exposure,under diverse environmental conditions for various periods oftime, and (2) to try to correlate the experimental in vitroresults with the composition of the materials as well as withtheir clinical performance. Results of this investigation shouldestablish which ingredients of the composites are responsible forcolor instability. Such data should assist in the development ofmore color stable resins.

Accomplishments

The color stability of composites on exposure to irradiation by(1) Xenon lamp, (2) a standard RS light source and (3) elevated

5

temperature at 60®C, in air or water and under differentexperimental conditions for various time periods wereinvestigated. Composites studied included chemically and lightcured hybrid and microfiiled resins and a material copolymerizedby light and chemical aftercure. Color changes of compositesintensified on increasing the irradiation time. Exposure to thelight sources under similar conditions gave comparable results.Visible light cured materials were more color stable than twocommon chemically cured composites. Light shades yielded morevisible color changes than dark shades of the same brand. Therewas a significantly more severe discoloration of composites keptin water at 60 “C than for those stored in air. Comparison withresults in two clinical studies of color changes using two of thechemically cured composites investigated here, indicates thatthese restoratives, which pass the ANSI/ADA and the ISOspecification tests, discolor in clinical use after 2-3 years.

Because of the intense discoloration which had occurred,quantitative estimation of the color changes which took placeafter one year water storage proved difficult.

Analytical high pressure liquid chromatography identified themajor constituents of composite pastes.

A recently published manuscript [1] of this work includescomparison of our laboratory data with clinical studies of colorchanges of two of the chemically cured composites. The dataindicate that these two restoratives, which pass the ANSI/ADA or

ISO specification tests, cause discoloration in clinical use aftertwo to three years

.

PROGRESS REPORT

Phase I. Characterize Minor Components of the Composites.Correlate Aging Behavior with Presence of Specific Ingredients

Characterization of the minor components awaits access to a

preparative chromatograph. Such an instrument will allowseparation of quantities of these constituents sufficient foridentification by analytical techniques such as infrared or

NMR.

Phase II. If Clinical Experience with Modern Composites is

Favorable, then Formulate New Composites with Improved ColorStability and Aging Characteristics.

No work on this phase is anticipated in the near future.

References

[1] Brauer, G.M. Color Changes of Composites on Exposure to

Various Energy Sources. Dent. Mat. 4, 55-59 (1988).

6

C . Development of Radiopaque Copolymerie Denture Resins

Overview

The ever increasing use of plastics in dentistry makes it

desirable and often mandatory that the materials have adequateradiopacity to detect their presence in various environments [1].

Radiopacity is an important requirement for medical and dentalimplants or devices that may be ingested accidentally. Thecommonly available high molecular weight plastics are composed of

elements of low atomic number. The low cross-sectional electrondensity of the polymer chain makes them radiolucent to x-rayimaging techniques. Attempts to render plastics radiopaque havetaken the following approaches: (1) incorporation of radiopaquemetals such a lead foil, gold or silver alloy [2-4]

, (2) additionof heavy metal salts such as barium sulfate, barium acrylate,bismuth subnitrate or yttrium fluoride as fillers [5-8]

,

or (3)

incorporation of an element of high atomic number into a silanizedglass used as reinforcing filler of a composite resin [9-12]

,

(4)

addition of halogenated saturated or unsaturated compounds such as

tetrabromoethane or aliphatic bromoacrylates or methacrylates to

the uncured resin [13,14]. Permanent radiopacity can also beachieved by entrapping polymer salt complexes such as thoseprepared on chelating copolymers of methyl methacrylate with BaBr2

to produce polymer salt complexes in an interpenetrating poly(methyl methacrylate) network [15,16], All these modificationshave certain disadvantages. Addition of metal or metal salts

causes stress concentrations at the interface between the insertand the resin which will weaken the materials. It also lowers the

mechanical properties (transverse, impact, compressive and tensilestrength) . Translucency of such radiopaque plastics is usuallylower than those containing no additives. Plastics withradiopaque glass ingredients are difficult to polish. Resins withhalogenated aliphatic ingredients discolor with time and are

unsuitable for many applications where esthetic characteristicsare important. The presence of aromatic monomers in the

polymerizable paste greatly reduces the storage stability of suchmixtures

.

The state of the art of radiopaque plastics for dentalapplications has been reviewed [1,2].

Introduction

At present, plastics used for most dental appliances and materialsincluding removable dentures and temporary crown and bridgematerials are radiolucent. Radiopaque denture base materialscombining adequate physical and esthetic properties with ease of

processing similar to the well accepted radiolucent acrylicplastics are not available commercially.

7

A desirable radiopaque plastic should have a homogeneouscomposition, with excellent mechanical, thermal and opticalproperties and high imaging characteristics so that its outline(including details) is readily visible in various environments onminimum exposure to x-ray radiation or ultrasonic waves. Theseproperties should not deteriorate on aging in the surroundingenvironment

.

Monomers with a high percentage of atoms of high molecular weightsuch as pentabromophenyi or triiodophenyl methacrylate arecompatible with methyl methacrylate [17]. The pentabromophenyimethacrylate is commercially available and the triiodo derivative(TIPMA) has been synthesized in this laboratory. The rate andkinetics of polymerization of monomer -polymer dough is not alteredgreatly by the addition of these halogenated compounds . Additionof 10 to 15 percent of the brominated monomer to methylmethacrylate results in radiopaque polymers. Such compositionsare more homogeneous than filler-containing compositions and havehigher strengths than commercial bone cements with barium sulfate

[17].

Objective

To develop a clinically useful radiopaque denture resin-based onhalogenated aromatic methacrylate copolymers.

Accomplishments

Compositions containing 10% and 15% pentabromophenyi methacrylate(PBPMA) or triiodophenyl methacrylate (TIPMA) in the liquid weremixed with commercial polymer powder. Resins with 15% halogenatedmonomers showed good radiopacity. Water sorption or watersolubility of the cured resin was not changed on addition ofhalogenated monomer. The PBPMA containing resin passed the colorstability test whereas material with TIPMA gave a perceptiblecolor change. Storage stability of liquids prepared with somebatches of PBPMA did not pass the 60°C storage stability test.

To overcome the deficiencies suspension polymer made up from 10%

pentabromophenyi methacrylate and 90% methyl methacrylate wassynthesized employing a commercial procedure. Using a doughprepar 'rom methyl methacrylate monomer and the suspensionpolymer powder cured denture base specimens with different powder-liquid ratios were evaluated. The radiopacity of the curedmaterials was excellent. The physical properties such as

transiucency,water sorption and color stability of the material

were good. The commercial methyl methacrylate liquid employed in

this formulation passed the thermal stability specification for

denture base polymers. To improve the mixing characteristics,additional quantities of pentabromophenyi methacrylate have beensynthesized. Difficulties were encountered in preparing powder

8

particles of the proper particle size and with a minimtain degree ofcrosslinking. Such powders are necessary to obtain mixes withdesirable working characteristics and short doughing and settingtime. To prepare compositions with optimal clinical desirableproperties,, detailed studies of the effect of plasticizer andcomonomers to increase swelling behavior of the powder will haveto be conducted.

PROGRESS REPORT

Phase I. Using Commonly Accepted Testing Procedures, Study the

Curing Characteristics and the Physical, Chemical, Esthetic andthe Radiopaque Properties of Self-Curing Denture Base MaterialsPrepared from Copolymers of Pentabromophenyl- and TriiodophenylMethacrylate and Methyl Methacrylate Copolymers.

If we are successful in synthesizing non-crosslinked powders,the properties of denture base materials prepared from themwill be evaluated.

Phase II. If the Properties of Formulation Studied in Phase I

Pass ail Requirements of the ANSI/ADA Specification for DentureBase Resins, have Adequate Working Properties and Good EstheticCharacteristics, We will Determine the Biocompatibility of theAdded Monomer and of the Cured Denture Base Copolymer Resin.

Work on this phase may be initiated at a later date.

References

[1] Council on Dental Materials, Instruments and Equipment (preparedby G.M. Brauer) . The Desirability of Using Radiopaque Plastics in

Dentistry: A Status Report. J. Am. Dent. Assoc. 102 347-349

(1981) .

[2] Bursey, D.C. and Webb, J.S. Incorporation of Radiopaque Materialsinto Denture Plastics. U.S. Armed Forces Med. J. ]A 561-566

(1960) .

[3] Atkinson, H.F. Partial Denture Problems: The Lack of Radio

-

opacity in Methylmethacrylate. Aust. J. Dent. ^ 349-351 (1954).

[4] Holden, M.H. The Radiolucent Plastic Foreign Body. Dent. Pract.(Bristol) 18 209-211 (1968).

[5] Alvares,L.C. Radiopacity of Acrylic Resins. Oral Surg. 22. 318-

324 (1966)

.

[6] Saunsbury, P. Radiopaque Denture Resins. Dent. Pract. lU 243-244

(1964)

.

9

[7] Combe, E,C. Further Studies on Radiopaque Denture Base Materials.J. Dent. 1 93-97 (1972)

.

[8] Elzay, R.P. Pearson, G.O. and Irish, E.F. Clinical andHistologic Evaluation of Commercially Prepared Radiopaque DentureMaterial. J. Prosthet. Dent. 25(3), 251-257 (1971).

[9] MacCulloch, W.T. and Stafford, G.D. Radiopaque Denture BaseMaterials. Br. Dent. J. 131 22-24 (1971).

[10] Chandler, H.H.,Bowen, R.L. and Paffenbarger

,G.C. Development of

a Radiopaque Denture Base Material. J. Biomed. Mater. Res. 5 253-

265 (1971).

[11] Chandler, H.H.,

Bowen, R.L. and Paffenbarger, G.C. PhysicalProperties of a Radiopaque Denture Base Material. J. Biomed.Mater. Res. 5 335-357 (1971).

[12] Chandler, H.H.,

Bowen, R.L. and Paffenbarger, G.C. The Need forRadiopaque Denture Base Materials. A Review of the Literature.J. Biomed. Mat. Res. 5 245-251 (1971).

[13] Molnar, J.E. (To COE Laboratories, Inc.) U.S. patent 3,715,331,Feb. 6, 1973.

[14] Combe, E.C. Studies on Radiopaque Dental Materials. Dent. Pract.22 51-54 (1971) .

[15] Xia, D.W.,

Silberman, R.,

Cabasso, I. and Smid, J. Novel PolymerSalt Complexes for X-ray Diagnostics. Polym. Prepr. 2^(1) 72

(1985)

[16] Cabasso, I., Obligin, A.S., Smid, J., Rawls, A.R. and Zimmerman,B.F. Radiopaque Acrylic Resins. J. Dent. Res. 128 (No. 176),1987.

[17] Brauer, G.M.

,

Steinberger, D.R. and Stansbury, J.W. ModifiedRadiopaque Bone Cement with Low Exotherms. Biomaterials 84,

Transactions, Voi. 7, p.347.

D

.

Study of Adhesion and Adhesion Promoting Agents to Dentin

Overview

The main disadvantage of most dental restorative materials is the

lack of adhesive bonding to tooth structures in the oralenvironment. These materials are kept in place solely bymechanical interlocking to the cavity preparation. A restorativethat chemically bonds to tooth structure should inhibit the

formation of secondary caries since it would prevent percolationof microorganisms and liquids by sealing the marginal areas of the

10

restoration. With such a material, sealing of incipient cariouslesions could be realized. An adhesive cement could lead to lessinvasive cavity preparations resulting in decreased loss of soundtooth substance. An effective bone adhesive to join poi3rmer

implants to bone as well as a soft tissue adhesive to bond dentureresin or implants to this substrate also could find manyapplications in clinical dentistry.

Introduction

Many procedures to obtain permanent bonding to enamel or dentinhave been suggested. Acid pretreatment of enamel enhancesadhesion of restorative resins to enamel, but this is

contraindicated for dentin. There are many constraints inherentin the long term bonding to dentin [1-4] and the present adhesiveshave found limited clinical use for this application. Surfacegrafting of monomers to mineralized tissues such as bone can beaccomplished with persulfate [5] and to dentin with tri-n-butylborane oxide as initiator [6,7]. Zinc polyacrylates andglass ionomer cements adhere to enamel but only weakly to dentin.

Excellent adhesion to dentin by acrylic resins is obtained withisobutyl 2 - cyanoacrylate

,but the bond strength decreases on water

exposure [8]. A mixture of a phosphorus ester of 2,2-bis[(2-hydroxy-3 ' -methacryloxypropoxy)phenyl] propane (BIS-GMA) anddiluent monomer has been introduced commercially as a dentin-composite bonding agent [9] . Monomers with hydrophobic andhydrophilic moieties such as 4-methacryloxyethyi trimelliticanhydride (4-META) and the diadduct of hydroxyethyl methacrylateand pyromellitic anhydride (PMDM) bond dentin to acrylic resins

[10,11]. Most effective are successive treatments of the dentinsurface with oxalate, a 5 to 10 percent acetone solution of a

glycine derivative (adduct of N-phenyi or N-p- tolyiglycine andglycidyl methacrylate) followed by a 5 percent solution of PMDM inacetone [11]. Another good bonding agent to dentin is the mixtureof 2 -hydroxyethyl methacrylate and glutaraldehyde (Gluma) [12-19].Major disadvantages of the dentin bonding systems are (1) thetooth surface must undergo a number of pretreatments which are

time-consuming and may limit the cost effectiveness of the

process, (2) strict adherence to the protocol such as removal ofexcessive monomer and water from the treated tooth surface [11]

are mandatory to obtain optimum bonding, and (3) the

biocompatibility of the various reagents and compounds used inmany of these tooth treatment procedures has not been fullyexplored.

The state of the art of soft tissue adhesives has been summarizedby Manly [22] and Peppas and Buri [23]

.

Two difunctional monomers, m-isopropenyl-a.a-dimethylbenzylisocyanate (TMI) and 2 - isocyanatoethyl methacrylate (lEM) and i.e.

compounds possessing polymerizable double bonds and reactiveisocyanate groups, have been described in the literature [20,21].

These compounds react with reactive hydrogen groups such as the-OH, or -NH2 of the amino acids present in collagen, yieldingcovalent bonds with the tissue surface. The resulting adductpossesses -C = C- that copolymerize in the presence of other vinylgroups to yield side chains grafted onto the collagenous tissues.

Objectives

Efforts are directed at developing film- forming surfaces thatserve as adhesives or sealants and to synthesize compounds andformulations containing functional groups that are effectiveadhesion promoters to mineralized tissues. Specifically thesestudies will (1) determine the grafting efficiency of oligomersand polymers of TMI or lEM and methacrylate with pendantisocyanate groups with the aim of obtaining a durable adhesivebond to the dentin surface, (2) synthesize and evaluate the

adhesive properties of methacrylate esters from a homocyclictetracarboxylic dianhydride which is closely related to the

aromatic dianhydride used to prepare PMDM and (3) study the forcesneeded to fracture cemented surfaces and the relationship betweenretention of cemented crowns and the film thickness of the cementlayer.

Accomplishments

Oligomers (low molecular weight polymers) with pendant isocyanategroups have been synthesized from m-isopropenyl-a,a-dimethyibenzylisocyanate (TMI) and/or 2- isocyanatoethyl methacrylate (lEM)

,two

difunctional monomers (Fig. 1) and over 50 methacrylate or vinylmonomers. The compounds were characterized by physico-chemicalmeans. Most of the oligomers are liquids at room temperature,stable in air, have a MW range from 1400 to 2600 and an isocyanatecontent from 5% to 18%. Dilute solutions of the compounds,especially those with TMI or lEM and TMI and methacrylateconstituents yielded stronger, more permanent bonds to

glutaraldehyde treated bone than other tissue adhesives.Thermocycling in water for one week between 5“C and 55 °C did notdecrease adhesion indicating that exposure to water or thermalshock produced no deterioration of the bond. No correlationbetween bonding efficiency and -NCO content or increase in

molecular weight could be established. Tensile adhesion of humandentin joined to composite restorative resins by means of the

oligomers was similar to that of the best dental bonding agents.Vinyl monomers containing oligomers are preferable to bond dentinto dental composites. The oligomeric compositions are alsoexcellent tissue adhesives and provide a strong bond betweencollagenous substrates such as calfskin and cured denture baseresins

.

12

PROGRESS REPORT

Phase la. Synthesize and Characterize Oligomers with PendantIsocyanate Groups.

Fig. 2 shows the structure of typical oligomers S 3mthesized in

this study.

A series of over 20 new methacrylate oligomers containing pendantisocyanate groups were synthesized by reacting TMI and/or lEM inethoxyethyl acetate with vinyl monomers such as vinyl acetate,vinyl stearate, 5-vinyl-2-norbomene

,N-vinyl-2-pyrollidone or a-

methy1styrene

.

Table 1 lists representative oligomers synthesized, abbreviationsused, ratio of monomeric reactants, and percentage of isocyanateper molecule. The oligomers are stable at room temperature. Theywere characterized by IR for -NCO, ester, C=C and aromatic groups,and by their refractive indices. They have residual double bondsand a molecular weight low enough so that the compounds areliquids at room temperature and dissolve readily in esters andchlorinated hydrocarbons. High pressure liquid chromatography(HPLC) showed no residual monomer. GPC and intrinsic viscosity ofselected oligomers indicated a molecular weight range from 1400 to

2600. The synthesis of compounds with higher molecular weight wasaccomplished by reacting the respective monomers at lowertemperatures (60®C) for longer periods of time (48 to 60 hours)and using smaller quantities of AIBN initiators than previouslyemployed. The resulting compounds had higher intrinsicviscosities (indicative of higher molecular weight) ranging from0.05 dl/g for TMI-SMA to 0.14 dl/g for TMI-BMA as compared to theintrinsic viscosities of 0.024 to 0.036 dl/g for the oligomerobtained with reaction temperatures of 140 “C. The highermolecular weight materials had a slightly higher isocyanatecontent

.

13

MONOMERS

WITH

ISOCYANATO

GROUPS

om otr: s:

oC\J

>1Nc:(D

x:)

f-i

x:4-0

G)

B•H G)

I

« Cn3

yI

o BrH o

CO

C MG)

CX.

OSX.

CX0CO

M1

S

oII

oII

I

CG:x;

01

c\j

:x:

01010=0 CVO

I "X.0—0II

>)sz4-0

CDO4-0

rdOrd

CJ0in

I—

\

1

CVJ

G)4^cd

I—

1

c.ocd

x:4-0

(D

S

suM

oOJ

rx

-TMI

ncniffi 2

locMumi sTmcTous of AOHam oligohob

-H«*vCOOCH,CH,NCO COOR

lEH • HtthaeryUta Esttr

Tni-HcthacryUta Ester

IDl-HethicryUtf Ester-TWI

lEM-VINYL NONOMER-TMI

15

Isocyanate groups determined titrimetricaiiy orspectrophotometricaiiy ranged from 5% to 18% per molecule . Thespectrophotometric procedure to determine the isocyanate contentof compounds developed by us depends on the intensity of theinfrared absorption peak between 2251 cm' ^ and 2267 cm' ^ . It is

much more rapid than the conventional titrimetric technique. Asshown in Table 2 results of both procedures are in good agreement.The observed percentage of -NCO groups of many oligomers is

considerably lower than the theoretical amount, i.e. thecalculated value, assuming complete conversion of monomers to theoligomers. Higher molecular weight methacrylate esters are eithermore reactive than lEM or TMI or they sterically hinder the

incorporation of the -NCO containing monomer into the oligomerchain. However, TMI is more reactive than low molecular weightmethacrylates such as methyl methacrylate (MMA) yielding oligomerscontaining a larger concentration of -NCO groups than would beexpected. A manuscript describing the synthesis andcharacterization of the oligomers has been accepted forpublication in the J. Biomed Mater. Research.

Phase Ib. Prepare Adhesive Formulations and Evaluate Their Properties.

The tensile bond strengths of glutaraldehyde treated bovine bonecemented with various oligomers (synthesized in phase la) are

summarized in Table 3. Highest bond strength was obtained withadhesive formulations of lEM (9.9 MPa) and TMI (9.0 MPa) monomers,followed by those containing lEM-BMA-TMI and lEM-CHMA-TMI (8.5

MPa) with 17 other oligomers fracturing above an applied load of 6

MPa. Bond strength values between 9.9 MPa and 6.8 MPa are notstatistically significantly different (p<0.05). After 7-daythermocycling between 5'’C and 55 “C the joints cemented with lEM or

TMI monomer showed a slight decrease in bond strength whereas sixof the nine oligomers studied had a slightly improved adhesion.

16

TABLE 1

PROPERTIES OF IEM-METHACRYLATES (OR VINYLVTMI OLIGOMERS SYNTHESIZED

Monomeric components:® Abbreviation %NCOFound— Calcd

lEM-Methacrvlate (ma^ EstersIt

- butyl ma lEM-BMA 1.477 9.58 14.27It - i-bornyl ma lEM-BOMA 1.485 6.33 11.12" - dicyciopentenyloxethyl ma

TMI-Methacrvlate Esters ('ma')

lEM-QM 1.505 7.31 10.13

11 - butyl ma (1:1.4) TMI-BMA(1:1.4) 1.514 12.15 10.49It

- stearyl ma TMI-SMA 1.481 6.11 7.78lEM-Methacrvlate Esters ('ma)-TMI

It - lEM-butyi ma-TMI lEM-BMA-TMI(1.3:1. 4:1) (1.3:1. 4:1) 1.508 14.06 13.95

ft - stearyl ma-TMI lEM-SMA-TMI 1.485 8.64 12.08tt

- allyl ma-TMI lEM-AMA-TMI 1.515 14.84 17.40It - cyclohexyl ma-TMI lEM-CHMA-TMI 1.512 13.49 16.01tt

- glycidyl ma-TMI lEM-GMA-TMI 1.513 10.92 16.85It

- i-bornyi ma-TMI lEM-BOMA-TMI 1.499 10.64 14.51tt - dicylopentenyloxyethyl- lEM-QM-TMI 1.524 9.54 13.57

ma-TMItt

- 9-vinyicarbazole-TMI lEM-VC-TMI 1.563 16.27 15.28It

- styrene -TMI lEM-S-TMI 1.538 14.64 18.23tt - a-methylstyrene -TMI lEM-MS-TMI 1.526 13.65 17.70It

- vinyitoluene-TMI lEM-VT-TMI 1.536 16.09 17.70TMI -Vinvl MonomerTMI - vinyl acetate (1:2) TMI-VAc 1.526 16.42 11.24

ft - vinyl stearate TMI -VS 1.478 5.73 8.21ft - vinyl butyl ether TMI-VBE 1.525 16.22 13.93tt

- 5-vinyl-2-norbornene TMI-VNB 1.533 13.80 13.06tf

- 1 -vinylimidazole TMI -VI 1.533 12.78 14.22It

- a -methylstyrene TMI -MS 1.527 12.97 13.15IEM-Vinvl monomer -TMIlEM - vinyl acetate -TMI lEM-VAc-TMI 1.511 17.90 18.98

It- vinyl stearate -TMI lEM-VS-TMI 1.481 8.46 12.59

tt - 5-vinyl-2-norbornene-TMI lEM-VNB-TMI 1.527 14.58 17.62tt

- N-vinyi-2-pyrollidone-TMI lEM-VP-TMI 1/531 16.33 17.96

®Ratio of monomeric reactants is 1:1 or 1:1:1 respectively except whengiven in parentheses

.

^Determined tirimetrically

.

17

TABLE 2

ISOCYANATE CONTENT OF SOME OLIGOMERS

OLIGOMER-NCO CONTENT, %

FOUNDTitrimetric Infrared

CALCD

TMI -ETHYL MA 13.16 13.46 13.32" -STEARYL MA 6.11 6.05 7.78"

-VINYLNORBORNENE 13.80 13.30 13.06"-VINYL ACETATE® 16.42 16.21 11.24

lEM-BUTYL MA-TMI^ 14.05 13.70 13.95"-STEARYL MA-TMI 8.64 8.78 12.08"-CYCLOHEXYL MA-TMI 13.49 13.69 16.01

® Monomer ratio 1:2 ^Monomer ratio 1.3: 1.4:1

Thus,

exposure to water and thermal shock produced nodeterioration of the bond compared' to the loss in bond strengthexperienced with 2 -cyanoacrylate adhesives (8)

.

All of thetensile strength specimens employed broke cohesively, with manyfracturing within the bone. Generally, oligomers containing TMIor TMI and lEM and methacrylate ester constituents adhere morestrongly to bone than those synthesized from lEM or vinylmonomers. Bonding to bone is strongest with methacrylateoligomers with high MW ester groups. From the experimental datano correlation between bonding efficiency and isocyanate contentof oligomer could be established (Table 4) . Oligomeric adhesiveswith intrinsic viscosity values ranging from 0.05 to 0.14 dl/gbond at least as well as to bone than the lower molecular weightmaterials. These studies are summarized in Table 5. Thus, the

values for the bone to bone tensile adhesion of ail five recentlysynthesized adhesives (lEM-BMA-TMI

,lEM-MS-TMI, TMI-BMA, TMI-QM

and TMI-SMA) were slightly, although not significantly, higherthan those of similar oligomers of lower molecular weight.

The concentration or pH of the glutaraldehyde solution employed in

the pretreatment of the bone did not influence greatly thestrength of the bone joint. On varying the concentration of the

glutaraldehyde solution between 5% and 25% (Table 6) or changingits pH between 3 and 9 (Table 7) no significant change (p<0.05) in

the bond strength of bone specimens cemented together with TMI-BMAor lEM-BMA-TMI occurred.

18

TABLE 3

TENSILE BOND STRENGTH OF HUMAN DENTIN OR BONE JOINEDWITH ADHESIVE OLIGOMERS

Monomeric Components: 2-Isocyanatoethyl methacrylate (lEM),

methacrylate orvinyl monomer and/or m-isopropenyl-a,a-dimethylbenzyl isocyanate

Bond Strength of Joint Cemented with Oligomer MPaBone to Bone^ Dentin to compos ite°

Adhesive-S. 24 h in H,0 7d thermocvcledi 24 h in Hr.1

lEM 9.9 ± 1 . 4"

- 6.4 + 2. Ur

TMI 9.0 + 1.8 - 4.3 ± 1.1

lEM-BMA 6.9 + 2.0 7.5 + 1.6 4.4 ± 1,7

lEM-BOMA 2.8 ± 1.1 - 3.9 ± 1.4

lEM-QM 5.2 + 1.9 - 6.3 ± 3.0TMI-BMA(1:1.4) 7.7 + 1.1 7.9 + 1.7 3.6 + 1.1

TMI - SMA 7.5 + 1.3 8.7 + 1.5 3.9 + 1.3

IEM-BMA-TMI(1.3::1.4:1) 8.5 ± 2.2 7.4 + 1.6 3.9 + 1.6lEM-SMA-TMI 7.4 + 1.5 8.1 + 1.8 4.5 + 2,4lEM-AMA-TMI 6.8 ± 2.3 6.7 ± 1.5 4.9 + 2.2lEM-CHMA-TMI 8.4 2.6 8.2 + 1.4 2.3 ± 0.9lEM-GMA-TMI 4.7 + 2.0 - 5.4 + 2.6lEM-BOMA-TMI 7.0 ± 3.1 7.2 ± 1.5 3.3 ± 1.6lEM-QM-TMI 7.5 ± 3.0 8.4 + 1.5 3.8 ± 1.3TMI-VAc(l :2) 5.4 ± 1.9 - 5.4 ± 1.7TMI -VS 6.8 ± 3.0 - 3.1 ± 1.6TMI-VNB 6.5 ± 2.6 - 5.5 ± 1.8TMI -VI 4.6 ± 1.6 - 4.2 ± 1.2TMI -MS 6.1 ± 1.6 - 5.4 ± 2.0lEM-VAc-TMI 6.8 ± 1.0 5.2 ± 1.6lEM-VNB-TMI 6.3 ± 1.5 - 5.8 ± 2.2lEM-VP-TMI 5.5 ± 0.8 - 5.2 ± 1.4lEM-VC-TMI 5.4 ± 2.0 - 4.8 ± 2.0lEM-S-TMI 5.2 ± 1.8 - 4.5 ± 1.4lEM-MS-TMI 6.6 ± 1.6 - 5.7 ± 1.6lEM-VT-TMI 6.3 + 0.6 - 4.3 ± 1.1

GLUMA® 7.1 + 0.8 6.9 + 1.4 4.2 ± 2.7NPG-PMDM^ 3.0 + 1.5 - 3.3 ± 2.0Control^- 1.1 ± 0.6 0 w 14

-

: 0.3^For abbreviations refer to Table 1.

^Procedure of Brauer et al (3), Bone was treated with 5% aq.

glutaraidehyde . A 5% solution of the adhesive in CH2C1^ was applied,followed by application of bonding resin. Mean of 10 specimens ± standarddeviation."^Procedure of Lacefield et al (29) was used. Dentin was pretreated with 0.5MEDTA and 5% aq. glutaraidehyde. A 5% solution of the adhesive in CH2 CI wasapplied, followed by application of a bonding resin and composite. Mean of 5

specimens ± standard deviation'^Between 5“C and 55®C-540 cycles per day.® Glutaraidehyde -2 -hydroxyethy 1 methacrylate -following EDTA pretreatment.^Ferric oxalate + N-phenylglycine + dimethacryloxyethyl pyromellitate

.

® Adhesive

19

Oligomers with pendant -NCO groups prepared from TMI or lEM-TMIand vinyl monomers are more effective adhesives for cementingdentin to composites than those synthesized with methacrylateesters (Table 3). Dentin cemented to composite with lEM-VNB-TMI,lEM-MS-TMI, TMI-VNB, TMI-VAc or TMI -MS had bond strength varyingfrom 5.4 to 5.8 MPa. These values were higher than those foroligomeric adhesives with methacrylate ester as constituents. Twoother bonding agents suggested for dentin, Gluma and NPG-PMDM, hadbond strength of 4.2 MPa and 3.4 MPa respectively. Presence of

heterocyclic nitrogen containing vinyl monomers in the oligomericadhesive did not improve bonding. Analogous to the resultsobtained for bone, increase in the molecular weight of the

oligomers slightly improved bonding to dentin (Table 5)

.

The oligomeric adhesives also bond well to soft tissues. Thus,

two calfskin specimen joined together with TMI-BMA, after standingin air for 24 h, fractured only after application of a 25.0 kgload. On exposure to H2 O, the calfskin swelled causing a rapidloss of tear strength which made accurate bond strengthdeterminations impossible. However, these bonded specimens stayedintact on storage in water over a six month observationperiod. Calfskin cemented to acrylic denture resins with lEM-VT-TMI or 8 other oligomers, when tested after 24 h storage in air,

broke after applying a load in excess of 20 kg, with some of the

specimens fracturing within the denture base. The results are

given in Table 8.

20

TABLE 4

EFFECT OF ISOCYANATE CONTENT OF ADHESIVEON BOND STRENGTH

OLIGOMER -NCO BOND STRENGTHCONTENT BONE DENTIN

% MPa MPaTMI-VS 5.5 6.8 3.1lEM-SMA-TMI 8.7 7.4 4.5IEM-BMA(1:1.4) 12.0 7.7 3.6TMI-MS 12.8 6.1 5.4lEM-BMA-TMI® 13.9 8.5 3.9lEM-VNB-TMI 14.6 6.3 5.8lEM-VAc-TMI 17.1 6.8 5.2

® Monomer ratio 1.3:1. 4:1

TABLE 5

EFFECT OF MW OF ADHESIVE ON BOND STRENGTH

OLIGOMER INTRINSICVISCOSITY

TENSILE ADHESION TOBONE DENTIN

dl/g MPa MPaTMI-BMA® 0.069 7.0 + 1.5 3.8 ± 0.8

n tf 0.14 7.3 + 1.8 4,1 ± 1.1

TMI-QM 0.039 6.3 + 2.0 4.5 ± 1.1n fi 0.10 6.4 + 2.0 4.8 ± 1.0

lEM-MS-TMI 0.021 6.2 + 00 5.2 ± 1.2n ft It 0.11 6.3 + 1.9 5.6 ± 2.1

lEM-BMA-TMI^ 0.064 8.1 + 2.0 4.2 ± 1.5ft ft It 0.13 8.3 + 2.1 4.3 ± 1.4

^Monomer ratio 1:1 ^Monomer ratio 1.3: 1.4:1

TABLE 6

EFFECT OF CONCENTRATION OF GLUTARALDEHYDEON ADHESION TO BONE

OLIGOMERIC ADHESIVE: 5% lEM-BMA-TMI

CONC. OF AQ. GLUTAR- BOND STRENGTHALDEHYDE TENSILE SHEAR

MPa KG1% - 19

5% 8.5 ± 2.2 41

25% 8.7 ± 1.5 43

21

TABLE 7

EFFECT OF pH OF GLUTARALDEHYDE ON ADHESION TO BONE

CONCENTRATION OF AQ. GLUTARALDEHYDE: 5%

pH OF GLUTAR-ALDEHYDE

BOND STRENGTH* OF BONE CEMENTED WITHTMI-BMA lEM-BMA-TMI

MPa MPa3

5

7

9

6.6 ± 1.67.0 ± 1.46.9 ± 1.65.9 ± 1.9

6.5 ± 2.37.9 ± 1.88.0 ± 1.86.3 ± 1.9

*Mean of 10 determinations

Three talks were presented discussing these studies. A manuscriptsummarizing this research has been published, another manuscript"Oligomers with Pendant Isocyanate Groups as Tissue Adhesives II.

Adhesion to Bone and Other Tissues" has been accepted by the J.

Biomed Mat Research. Two other papers have been submitted forpublication. A patent for these adhesive oligomers has beenapplied for.

Phase Ic. If Studies Conducted in Phase la and Ib are Successful, InitiateCooperative Projects to Study Biocompatibility of These Adhesive Systems andTheir Application in Dental Practice.

The biocompatibility of three oligomers furnished by thislaboratory is being studied at Northwestern University, theMedical College of South Carolina and the Dental School,University of Regensburg, Germany, Dept. of Orthopaedics,University of Calgary, Canada and the University of Liverpool,England. Over 300 bone specimens cemented with the threeoligomers have been prepared and were implanted by Dr. Kafrawy at

the University of Indiana to study tissue reactions of the curedadhesive. Feasibility of using these materials as soft tissueadhesives is being explored at the Ohio State School of Dentistry.

Studies conducted by Dr. Schmalz, University of Regensburgindicate that the undiluted oligomers containing lEM are cytotoxicwhereas those with TMI show only very moderate cytotoxicity.Based on these results different TMI monomers that should haveminimum cytotoxicity have been prepared and sent for evaluation.

22

TABLE 8

Relative Adhesive Shear Strength of Soft Tissue ('Calfskin) Cemented to CuredDenture Base with OligQmers

Specimens stored in air for 24h

Oligomeric Adhesive— Load at Fracture in kgf

TMI-•SMA 18,.16 + 4,.71

TMI-•VS 21,.09 ± 6,.57

lEM-•BMA-TMI(1.3:1.4:1) 23,.70 + 5,.87

lEM-•CHMA-TMI 22,.44 + 8..92

lEM-•VAc -TMI 15,.67 + 5.,82tlEM- VS- TMI 20,.15 + 5.,26^

lEM-VBE -TMI 13,.79 + 6.,51tlEM-VNB -TMI 20,.36 + 4.,72

lEM- VP- TMI 17,.18 + 5.,15|

lEM- VC- TMI 18,.97 ± 4,.71^

lEM- S-TMI 14,.69 + 2..251

lEM- MS- TMI 21,.50 ± 3,.48

lEM- VT- TMI 25,.62 + 5,.03

For abbreviations refer to Table 1

f Mean of 10 determinations ± standard deviation

I Mean of 5 determinations ± standard deviation

Recent studies have shown that oligomers with lower isocyanatecontent or higher molecular weight than those originally preparedhave the same adhesive properties as the oligomers initiallyS 3mthesized. The higher MW compounds have lower volatility andboiling points, should diffuse more slowly into tissues and mayhave lower irritational properties

.

The adhesives described in this study comprise a generic series ofoligomers. Thus, it is possible to adjust many variables such as

molecular weight, isocyanate content, volatility, viscosity,working properties and rate of diffusion of adhesive into tissuesand to obtain molecules having the desired physical, chemical andbiological properties or to modify, if necessary, any adversetissue reactions. Subject to their acceptable biocompatibilitythese oligomers could find applications in medical, dental orveterinary procedures

.

Phase II. Synthesize the Dimethacrylate Ester of 5-(2,5-Dioxotetra-Hydrofuryl) -3-Methyl- 3 -Cylohexene-1 , 2-Dicarboxylic Anhydride and Determineits Adhesion to Dentin.

23


This dimethacrylate has a more non-polar homocyclic nucleuscompared to the aromatic groups in 4 -META or PMDM. It wassynthesized at room temperature with hydroxyethyl methacrylate inCH2 CI 2 and dimethylaminopyridine accelerator. The viscous productadhered well to ferric oxalate treated bone and composite. Thebond strength appeared to be lower than that of the isocyanatecontaining oligomers (phase I). Thus, investigation with theoligomers described in phase I was emphasized during this period.

Phase III. Study the Effects of Film Thickness on the Retention of Cements.


Film thickness (FT) and retention of zinc phosphate,polycarboxylate and ionomer cements as a function of powder- liquid(P/L) ratio was determined for both nonvented and vented crownsusing the parallel plate specification test. FT increased, withthe P/L ratios. Cementation of vented crowns yielded the thinnestFT usually followed by that obtained by the parallel plate methodand the nonvented crown.

The effect of P/L ratio on FT or retention was more dependent onthe composition of the brand than on the type of cement. For somepolycarboxylate or ionomer cements, retention decreased withincreasing film thickness. Little retention of well- fitted crownswas found for zinc phosphate powder-water mixes, although suchretention had been suggested previously [25].

The P/L ratios suggested in the manufacturer's instructions didnot always yield the maximum retention for either vented or

unvented crowns. Venting of prepared crowns is indicated sincethis procedure usually increased their retention and producedsmaller changes in film thickness on changing the P/L ratios. Apaper describing this study has been published.

References

[1] National Institute of Dental Research: Adhesive Restorative DentalMaterials - II . Public Health Service Publication No. 1494, U.S. Govt.

Printing Office, Washington, DC (1966).

[2] Brauer, G.M. The Chemistry of Biosurfaces. M.L. Hair, ed. Vol. 2,

Marcel Dekker, NY, NY, p. 731-800 (1972).

[3] Brauer, G.M. Scientific Aspects of Dental Materials. J.A. vonFraunhofer, ed.

,

Butterworths,London, p. 49-96 (1975).

24

[4] Causton, B.E. Biocompatibiiity of Dental Materials, D.C. Smith andD.F. Williams, ed. Vol. II, p. 125-141, CRC Press, Inc., Boca Raton, PTL

(1981) .

[5] Brauer, G.M.,Termini, D.J. and Levy, S.M. J. Dent. Res., 646-659

(1977).

[6] Sanjo, D. J. Dent. Res., 50, 60 (1971).

[7] Miura, F.,

Nahagawa, K. and Masuhara, E. Am. J. Orthod.,

35

(1971).

[8] Brauer, G.M.,Jackson, J.A. and Termini, D.J. J. Dent. Res., 1900-

1907 (1979).

[9] 3M Dental Products Department, Pamphlet, Scotchbond Dental AdhesiveSystem (1982).

[10] U.S. Patent, 4,148,988 (April 10, 1979), E. Masuhara, N. Nakabayashiand J. Takeyama (to Mitsui Petrochemical Industries Ltd.).

[11] Bowen, R.L.,Coble, E.N. and Rapson, J.E. J. Dent. Res., 1070-1076

(1982)

.

[12] Munksgaard, E.C. and Asmussen, E. J. Dent. Res. 63^, 1087-1089 (1984).

[13] Munksgaard, E.C.,

Itah, K. and Jorgensen, K.D. J. Dent. Res. 144-

146 (1985).

[14] Hansen, E.K. and Asmussen, E. Scand. J. Dent. Res. £3, 280-287 (1985).

[15] Munksgaard, E.C. and Asmussen, E. J. Dent. Res. 63, 1087-1089 (1984).

[16] Munksgaard, E.C. and Asmussen, E. Scand. J. Dent. Res. 93, 463-466(1985) .

[17] Hansen, E.K. and Asmussen, E. Scand. J. Dent. Res. 93, 280-287 (1985).

[18] Munksgaard, E.C. Irie M. and Asmussen, E. J. Dent. Res. 64, 1409-1411(1985)

.

[19] Munksgaard, E.C. Itoh, K. Asmussen, E. and Jorgensen, K.D. Scand. J.

Dent. Res. 93, 377-380 (1985).

[20] Thomas, M.R. Isocyanatoethyl Methacrylate: Org Coatings and PolymerSci. Proc. Preprints of papers presented by the Div. of Org. Coatingsand Plastics Chem. at the 183rd ACS Meeting, Las Vegas, Nev.

,506-

513 (1982).

[21] Dexter, R.W.,

Saxon, R. and Fiori, D.E. Polvm Mater Sci and Eng . Failmeeting 1985, Chicago, IL, Sept. 53., 534-539 (1985)

25

[22] Manly, R.S., Ed. **Adhesion in Biological Systems '* Part III Adhesivefor Soft Tissue Academic Press, New York, 1970, pp. 153-221.

[23] Peppas, N.A. and Buri, P.A. J. Control Release, 2, 257-275 (1985).

[24] Lee, L.,Swartz, M.L. and Culp, G. J. Dent. Res., 211-216 (1969).

[25] Paffenbarger ,G.C.

,Sweeney, W.T.

,and Isaacs, A. JADA 21,, 1907-1924

(1934)

.

[26] Kemper, R. and Kilian R. J. Dent. Res. 5^, 148 (Abstr No. 308) 1975.

[27] Lacefield, W.R.,

Reindl, M.C. and Retief, D.H. J. Prosth. Dent. 53 .

194-198 (1985).

[28] Bowen, R.L. J. Dent. Res., 690-695 (1965).

Publications

lizuka, H. Brauer, G.M.,

Rupp, N. Ohashi,

M. and Paffenbarger, G. ForcesFracturing Cements at Die Interfaces and Their Dependence on Film Thickness.Dent. Mat., 3, 187-193 (1987)

Brauer, G.M. Color Changes of Composites on Exposure to Various EnergySources. Dent. Mat. 4, 55-59 (1988).

Brauer, G.M. Vanillate and Syringate Cements. Trends and Techniques ^ (No.

1), 6 (1988).

Brauer, G.M. and Lee, C.H. Methacrylate Oligomers with Pendant IsocyanateGroups as Tissue Adhesives. Poly. Mater., Sci. and Eng. 397-401 (1988).

Manuscripts Under Review or Accented for Publication

Brauer, G.M. and Lee, S. Oligomers with Pendant Isocyanate Groups as TissuesAdhesives I. Synthesis and Characterization, (accepted, J. Biomed. Mat. Res.)

Brauer, G.M. and Lee, S. Oligomers with Pendant Isocyanate Groups as TissueAdhesives II. Adhesion to Bone and Other Tissues (accepted, J. Biomed. Mat.Res

.)

.

Brauer, G.M. and Lee, C.H. Oligomers with Pendant Isocyanate Groups as

Adhesives for Dentin and Other Tissues. (accepted, J. Dent. Res.).Brauer, G.M. Vanillate Cements. Update.

Keller, J.C., Hammond, B.H.

,

Kowalyk, K.K. and Brauer, G.M. BiologicalEvaluations of Zinc Hexyl Vanillate Cement Using Two in Vivo Test Methods,(submitted. Dent. Mat.)

26

Brauer, G.M. and Lee, S. Methacrylate Oligomers with Pendant Isocyanate Groupsas Tissue Adhesives. Progress in Biomedical Polymers, Gebelin, C.G. Ed,

Pergamon Press.

Patents Applied For

Brauer, G.M. and Lee, C.H. Oligomeric Adhesives.

Brauer, G.M., Stansbury, J.W. and Tesk, J.A. Radiopaque Polymers Useful as

Components for Radiopaque Materials

.

Contributed Talks

Brauer, G.M.,

and Lee, C.H. Properties of Oligomeric Adhesives with PendantIsocyanate Groups. lADR, Montreal 1988.

Lee, C.H. and Brauer, G.M. Oligomeric Adhesives from Vinyl Monomers, 2-

Isocyanatoethyl Methacrylate and TMI. lADR, Montreal 1988.

Brauer, G.M. and Lee, C.H. Oligomeric Adhesives from Vinyl Monomers withPendant Isocyanate Groups. Midatlantic Regional Meeting, American ChemicalSociety, Millersville

,PA 1988.

Brauer, G.M. and Lee, C.H. Methacrylate Oligomers with Pendant Isocyanate as

Tissue Adhesives. American Chemical Society. Los Angeles, Sept. 1988.

27

E. Monomers That Polymerize with Expansion

Introduction

A trait universally shared by conventional monomers in use todayis the volume contraction that accompanies their conversion topolymer. In dental composite applications, this polymerizationshrinkage can result in a number of defects including marginalgaps, loss of adhesion and interfacial debonding of the filler.These adverse effects can be countered by employing spiroorthocarbonates as ingredients in formulations since thesemonomers are capable of double ring-opening polymerization to givean expansion in volume.

Objective

This project represents an attempt to minimize, if not completelyeliminate, the stress- inducing polymerization shrinkage in dentaladhesive and composite resin materials through the synthesis andutilization of novel spiro orthocarbonate monomers which expandupon polymerization.

PROGRESS REPORT

Phase I. Synthesis

A spiro orthocarbonate monomer was synthesized which integratesall the advantageous elements discovered through previous workwith simpler model compounds. This new monomer, 2-methylene-8,9-benzo- 1 , 4 , 6 , 11 - tetraoxaspiro [4.6] undecane (I), has reactivitycomparable to acrylic monomers used in resin-based dental polymersby virtue of the direct substitution of oxygen on the double bond.The five-membered, unsaturated ring also promotes more efficientdouble ring opening during polymerization. The seven-memberedring component induces additional strain which contributes to ringcleavage and it also provides asymmetry to help minimize the

melting point of the monomer. The aromatic group serves to

stabilize the ring-opened radical and thus, it too encourages thisprocess. Additionally, the aromatic ring raises the glasstransition temperature of the resulting polymers and gives a UVand IR visible marker.

Phase II. Polymerization

The polymerization of monomer I has been conducted in bulk and in

solution at several temperatures. Under all conditions, completedouble ring opening has occurred. This is the first spiroorthocarbonate monomer able to provide efficient, low temperaturering opening by a free radical mechanism. This is obviously anadvantage for a monomer being considered for a dental application.The combination of reactivity, ring opening efficiency and the

relatively rigid structure of its polymers make I an excellent

28

candidate monomer for minimizing polymerization shrinkage in

dental resin systems. A calculation of polymerization expansionand the evaluation of I in resin formulations remains to be

completed.

Phase III . Formulation

The storage stability of a previously prepared spiro monomer (II)

which contains the unsaturated five-membered ring functionalitywas investigated in formulations activated for either light cureor chemical cure. The formulations were prepared by adding 25

wt.% of II to a control mixture comprised of ethoxylated bis-phenol A dimethacrylate and hexamethylene glycol dimethacrylate(9:1). The and NMR spectra were taken immediately aftermixing the experimental formulations . These were compared withthe analogous spectra obtained after four months of roomtemperature storage. There was no indication of significantchange in any of the components. This relatively hydrophobicresin was chosen to minimize the potential for hydrolysis of the

spiro orthocarbonate. A parallel study involving II in

conjunction with a more hydrophilic BIS -GMA/triethylene glycoldimethacrylate resin is currently underway.

Phase IV. Evaluate the Properties of Polymers Formulated in PhaseIII

Composites prepared from the formulations described in Section III

provided cured materials with acceptable mechanical strengthproperties. The polymerization shrinkage results as well as

adhesive properties of these formulations will be conducted in the

near future

.

Phase V. Evaluation of Biocompatibility of Formulations Selectedfrom Phase IV.

This work awaits completion of Phase IV.

Invited Talks

Stansbury, J.W., and Bailey, W.J. Synthesis and Free RadicalPolymerization of Unsaturated Spiro Orthocarbonates. Mid-AtlanticRegional Meeting of the American Chemical Society, High PerformancePolymers Symposium, Lancaster, PA, May 1988.

Stansbury, J.W. and Bailey, W.J. Evaluation of spiro ortho -carbonatemonomers capable of polymerization with expansion as ingredients in

dental composites. American Chemical Society, Symposium on Progress in

Biomedical Polymers. Los Angeles, CA, Sept. 27, 1988.

29

properties of a poiyfluorinated prepolymer muitifunctionai urethanemethacrylate. Polym. Mat. Sci. Eng. ,

Proceedings of the ACS Divisionof Polymeric Materials, Vol. 59, 388-396, 1988.

Stansbury, J.W., Bailey, W.J. Evaluation of spiro orthocarbonatemonomers capable of polymerization with expansion as ingredients indental composite materials. Poly. Mat. Sci. Eng., Proceedings of the

ACS Division of Polymeric Materials, Vol. 59, 1988.

30

F. Improvement of Dental Composites. Sealant Cement and AdhesiveMaterials

Overview

The quest for a durable, esthetic, adhesive and biocompatiblematerial suitable for the restoration of lost tooth structure haslong challenged dental materials researchers. A significant step

toward the realization of this goal was the development of resin-based dental composites which overcame many of the shortcomings ofthe silicate cements (purely inorganic composites) and unfilledresin restoratives (purely organic composites based on methylmethacrylate and its polymers) . The synthesis of BIS-GMA byBowen, ushered in the modem era of resin-based dental compositerestorative materials and also other resin-based dental materials[1,2]. The essential components of dental composites are: (1) a

resin system comprising one or more vinyl monomers which onpolymerization forms the matrix or continuous phase, (2)

reinforcing fillers such as radiopaque glasses, quartz, minerals,ceramics, organic and hybrid organic- inorganic powders of varioussizes, size distributions and shapes constitute the dispersedphase, (3) an interfacial phase for bonding the continuous anddispersed phases, derived from vinyl silanes, titanates, i.e.,

coupling agents, (4) a polymerization initiator system effectiveunder ambient conditions, and (5) stabilizers for optimizingstorage stability and also preserving the chemical stability ofthe hardened restoration. Unlike glass ionomer cements which bondto tooth structure, current resin-based composites are non-adhesive in nature [3,4].

However, the acid-etch technique (Buonocore),

in most situations,provides an effective micromechanical mechanism for bonding dentalcomposites to enamel [5,6]. Bonding to dentin is a morechallenging problem but recent developments appear to be yieldingeffective coupling agents for this substrate as well [7-16].Efforts to enhance the durability and range of applications (e.g.,

posterior as well as anterior fillings) of dental compositesinclude optimization of the types, sizes, shapes and volume of thedispersed phase, reductions in the solubility parameter, residualvinyl unsaturation, and polymerization shrinkage of the resinphase, and the development of more effective interfacial bondingphases

.

Dental sealants have similar compositions and chemistry but are

unfilled or only lightly filled and usually contain a higherproportion of diluent monomer(s) . Similar resin-based materialsalso are widely used in other applications (e.g., adhesives , corebuild-up and crown and bridge materials, laminating veneers,etc

.)

.

31

Dental cements, which also have a composite nature, find use in a

wide variety of dental applications. In restorative dentistrythey are employed as temporary, intermediate, and (in the case ofglass ionbmer cements) permanent filling materials. Theirchemistry of hardening involves a series of acid-base reactionsinvolving ion- exchanges that result in the development of a matrixinto which are imbedded partially reacted basic filler particles

[3] . An ion- exchange mechanism involving polyelectro lyte cements(e.g., glass ionomer) and mineralized tissue also may explaintheir adhesion to tooth structure [4] . Other mechanisms forbonding to tooth structure will be discussed in Part IV, pp. 56-57

[5-16].

Two types of dental cements can be distinguished depending ontheir water content: (1) those that are aqueous based (e.g., zincphosphate, poiycarboxylate

,glass ionomer) and in which water

plays a role both in their setting and in the development of theirmolecular and micro structures, and (2) those that are relativelynon-aqueous in nature, although catalytic amounts of water or

other polar compounds (e.g., acetic acid) are needed to achieveclinically acceptable setting times (e.g., ZOE, EBA, HV-EBA, dimeracid, etc.)

This section is divided into four distinct parts:

I. Improvement of Dental Resin Systems for Composites andSealants

.

(Synthesis, Formulation and Evaluation)II. Improvement of Dental Cements.

(Synthesis, Formulation and Evaluation)III. Improvement of Interfacial Bonding Systems and Fillers for

Composites and Cements. (Filler Portion of Project is New)

(S 3mthesis, Formulation and Evaluation)IV. Bonding of Low Surface Energy Resin Systems to Tooth

Structure. (New Project) (S3rnthesis, Formulation andEvaluation)

I. Improvement of Dental Resin Systems for Composites and Sealants

Objective

The goal of this research task is to enhance the durability ofdental composite, sealant, cement and adhesive materials throughthe use of low- shrinking, but highly thermosetting and hydrophobicresins

.

Background

Recent research has indicated that the oral environmentalresistance (OER) of resin-based dental materials is a significantfactor in determining their in vivo performance and ultimateservice life both in relatively stress-free as well as in stress-

32

bearing applications [17-19]. The continual sorption of water andother intraoral substances can promote plasticization of the

organic matrix, which can be viewed as the first line of defenseof composites and related resin-based dental materials against the

constant assaults of the oral environment. Ultimately, this

chemical softening process can lead to degradation reactions notonly in the polymeric binder and critical interfacial phase but,

in some cases, even in filler phases as well [20-23]. Two relatedapproaches have been initiated in our laboratories as methods to

enhance the OER of these materials. One involves the use ofhydrophobic resin systems that have solubility parameters lower

than that prevalent in the oral environment [24-26] . The otheris aimed at increasing the degree of poi

3nnerization and the

crosslink density of resin systems by methods compatible with the

clinical situation [26-29]. The first approach involves the

synthesis and formulation of resins that yield polymers ofsignificant fluorocarbon or siloxane content to confer on dentalmaterials the necessary hydrophobicity (by lowering solubilityparameters) to resist the detrimental effects of the oralenvironment [24,25,30,31]. The second approach, which also maycomplement the first approach, involves the synthesis andformulation of multifunctional methacrylates and/or chain transferagents as well as other types of network forming agents that canaugment the degree of cure and crosslink density [27-35].

Emphasis also is directed to monomers that have maximal cure andminimal polymerization shrinkage [35]

.

An ancillary part of this

approach involves the development of more efficient, color stableinitiator systems for ambient polymerizations of dental resins ina more uniform fashion [27,28].

PROGRESS REPORT

Phase I. Synthesis and Formulations

(a) Synthesis of oligomer and diluent multifunctional monomers of

higher fluorine content, workable viscosities and greaterflexibility.

This part of Phase I was deferred due to technical problemswhich affected the performance of the High Performance LiquidChromatograph which is essential for establishing the purityand aiding in the characterization of monomers such as

2 , 2 , 3 , 3 , 4 , 4-hexafluoro- 1 , 5-pentamethyiene dimethacrylate[30].

(b) Formulation and evaluation of dual-cured composites.

There is evidence [17-20] that a significant, contributoryfactor involved in the failure of dental composites is the

susceptibility of their polymeric matrices and interfacialphases to swelling and possible chemical degradation inducedby oral fluids. Accordingly, it is important to develop

33

polymeric systems which are highly resistant to intraoralfluids, which include organic components as well as theubiquitous water. In order to improve the resistance to

degradation from the intraoral environment, the dentalpol3niier should have the following characteristics;

(1) a low solubility parameter

(2) a high glass transition temperature (Tg) as a resultof a high degree of cure and crosslink density in theresin

(3) a low pol3nnerization shrinkage

(4) and the ability to bond effectively to the fillerphase via interfaciai coupling agents and/or othermodes interfaciai attachment.

The following resin formulation was designed to optimize (1)

(2) and (3). PFMA (Fig. 1) was blended with three diluentmonomers, the bis (methacryloxyethyl) ether of resorcinol(MER)

, 1 , 10-decamethylene dimethacrylate (DMDMA) and ca-

methylene 7 -butyro lactone ,ca-MBL, [36,37] shown in Fig. 2

along with a photoinitiator system consisting ofcamphorquinone (CQ) and ethyl 4-N,N-dimethylaminobenzoate(4EDMAB) . The ingredients used in the formulation of thesePFMA dual-cured materials (PFMA-DC) are listed in Table 1.

The compositions of the PFMA-DC resin and composite are givenin Tables 2 and 3, respectively. Both are two part systems.Mixing equal parts of resin 1 and resin 2 of Table 2 resultsin a conversion of 65% compared to 53% for the same resinsystem that is cured only by visible light initiatedpolymerization. The diametral tensile strengths of theircorresponding composites was 46 and 39 MPa, respectivelyKnoop hardness determinations showed similar differences withthe PFMA-DC composites having the higher values. A similarstudy was done with a commercial resin (NCO, Caulk/Dentsply)and its composites. The results indicate that the dual-curedresin had the higher conversion (77% vs 71%) and its

corresponding composite the higher diametral tensile strength(59 vs 47 MPa) than usual light cured materials. Two papersbased on these studies were presented at the 1988 lADRmeeting.

(c) Synthesis of highly fluorinated multifunctionalmethacrylates based on hexachlorocyclophosphazene has

been dropped due to similar work being done elsewhere[33-35]

.

(d) Silyl ether derivatives.

No work was done on this project in FY88.

34

(e) Synthesis of hydrophobic urethane derivatives of BIS-GMA.

No work was done on this project in FY88.

(f) Additives to improve the physical and chemicalperformance of resin-based materials by increasing thedegree of polymerization and crosslink density.

In a prior investigation, the incorporation of a-

methylene butyrolactone I in small proportions to a BIS-GMA/triethylene glycol dimethacrylate controlformulation provided an improved degree of conversionand an increase in mechanical strength properties. Thesynthesis of high molecular weight methylene lactoneswas undertaken to reduce the magnitude of the shrinkageassociated with the high conversion polymerization ofthese very reactive monomers . To achieve thisbenzaldehyde and cyclohexanone were converted to thecorresponding methylene lactone derivatives II and III,

respectively, in a one step reaction. Thepolymerization of the available unsaturated lactonesprovides linear polymers which could then be rankedaccording to solvent resistance. Polymers of I wereonly slightly swollen by DMSO at >100 °C whereasmaterials from II could be dissolved in warm DMF. Thepolymers from III were the least resistant and weresoluble in chloroform. The synthesis of a di£unctionaimonomer based on the a -methylene lactone group is

underway.

35

CF3 CF3 CF3I I I

C-OCHa-CH-CHs-Cr-CH^CH-CHgC-OCHa-CH-CHa-O

CF3 0 CF3 CF3 0I I

-

c=oI

C-CH3II

CH2

c=oI

C-CH3u

CHa

MW (Repeat) = 1032Av. MW s 10,320 (N s 10)

Fig. 1. Chemical structure of the base oligomer PFMA.

36

CH 30I «

CHg *C—COCH gCH 2O

0 CH3

0CH2CH20C-C'CH2

MW = 334( a )

0 0u u

CH2=C-C-0-( -CH2)io’OC-C=CH2

CH3 CH3

MW = 310(b)

ch2=c-ch2"^"Q~9=Q

MW = 98(c)

Fig. 2. Chemicai structure of the diluent monomers used in the PFMA-DC resin,(a) HER, (b) DHDHA, and (c) a-MBL.

37

Table 1

Ingredients of the PFMA-DC Composite

Abbreviation Name Source

PFMA Poly (fluoromethacrylate) NBS

MER 1 ,3 -bis (2' -methacryloxyethoxy)

benzenePRF* (ADA) -NBS

DMDMA 1,10 decamethylene dimethacrylate EsschemEssington, PA

ot-MBL a-methylene- 7 -butyrolactone Aldrich Chemical CoMilwaukee, WI

4EDMAB ethyl 4-N,N, -dimethylaminobenzoate Aldrich Chemical CoMilwaukee

,WI

CQ Camphorquinone Aldrich Chemical CoMilwaukee

,WI

BPO Benzoyl peroxide Lucidol/PenwaltBuffalo, NY

FFQ Fine fused quartz Caulk/Dentsp lyMilford, DE

A-174^« 3 -methacryloxypropyl-trimethoxys i 1ane

Union CarbideNew York, NY

Paffenbarger Research Foundation (American Dental Assoc.)

38

Table 2

Composition of the PFMA/DC Resin

Component Resin 1

Wt. %

Resin 2

Wt. %

PFMA 68.57 68.55

MER 18.07 18.07

DMDMA 10.03 10.03

a-MBL 1.37 1.374EDMAB 1.56 -

CQ 0.40 -

BPO - 1.98

Total 100.00 100.00

Resin 1/Resin 2 (wt/wt) = 1.00

39

This should yields monomers which can polymerize andcrosslink to provide materials with high degrees ofconversion and extreme resistance to chemical softening.

IIIII

An outgrowth of the work on methylene lactones has leadto the formation of a new class of difunctional monomers(IV) . The reaction between formaldehyde andconventional acrylate monomers gives a diacrylate withpendant ester groups which can be varied to alterproperties of the monomer. The monomer (R=ethyl)obtained from ethyl acrylate can be used as a diluentcomonomer while the bulkier monomer (R-phenethyl) basedon phenethyl acrylate is better suited as a base monomerin resin formulations. Due to the position of thedouble bonds, these monomers can polymerize by a highlyefficient cyclization process which introduces a six-membered ring into the polymer backbone (V) . Thismechanism allows polymerizations at near ambientconditions to proceed to high degrees of conversion(R=ethyl; 78%, R=phenethyl; 100%) while producingpolymers containing rigid units which should convey goodmechanical strength properties to materialsincorporating these monomers

.

40

Table 3

Composition of the PFMA-DC Composite

Component Wt.%

Resin:

PFMA 69.25HER 18.25DMDMA 10.13a-MBL 1.384EDMAB 0.79CQ 0.20

Total Resin 100.00

Filler:

FFQ 98.50A-174 0.50BPO 1.00

Total Filler 100.00

Filler/resin (wt/wt) =2.20

41

(g) Improved initiator systems for dental resins

Several types of dual-curing initiator systems wereinvestigated in an attempt to overcome the limited depthof cure of conventional visible light initiator systemsbased on camphorquinone and tertiary amines . Two partsystems utilizing benzoyl peroxide as the chemicalinitiator yield better cured composites than thecorresponding one part, visible light activatedcomposite as previously described. Two part dual-curingsystems suffer from the usual deficiencies of twocomponent chemically activated resin systems; namely,limited working time and the introduction of porositydue to component mixing. It would be desirable to havea one component resin based dental material thatpossessed dual-cure characteristics, i.e. photochemicalplus chemical initiating potential. The problem hasbeen on how to stabilize such a system. In this studywe explored the feasibility of using PFMA based resins(Fig. 1), which have the capability of dissolving moreoxygen than conventional dental resins, as means ofimproving the storage stability of dual-curable, one-part resin based dental materials. BIS-GMA resinscontaining CQ, 4EDMAB and 1% b.w. BPO have poor storagestability at room temperature/about 30 minutes) With a

PFMA resin the same initiator system did not causegelation for 3-4 hours. The substitution of the morethermally stable peroxides such as t-butyl perbenzoateor cumene hydroperoxide improves the storage stabilityof both resin types towards premature gelation.Composites based on the PFMA resin shown in Table 2

containing, CQ(0.4%) and 4EDMAB (1.6%) and either t-

butyi perbenzoate (2%) or cumene hydroperoxide (2%) haddiametral tensile strengths of 43 and 45 MPa,respectively. With the resins containing CQ, 4EDMAB andcumene hydroperoxide, it was noted that visible lightphotoactivity decreased with time presumably due to the

oxidation of CQ by the hydroperoxide since the resin wasreactivated by the addition of CQ. It is not known at

this time if a similar but slower degradation of CQoccurs with resins containing CQ and peresters. Whilethis study did not result in a sufficiently stable,single paste, dual-curable composite, it did demonstratethat dual -cure initiator systems improve the conversion,uniformity of cure and related properties of resin baseddental materials. A paper based on this work waspresented at the 1988 lADR meeting.

42

II. Improvement of Dental Cements

Overview

Although used in relatively small quantities, dental cementsare essential in a number of dental applications as: (a)

temporary, intermediate and, in some cases, more permanentrestoratives, (b) cavity liners and bases, (c) luting agentsto bond preformed restorations and orthodontic devices, (d)

pulp capping agents and endodontic sealers, and (e)

impression pastes. This wide range of dental uses makes it

virtually impossible for one type of cement to have all the

necessary properties demanded in these diverse applications.Therefore, there is a need to develop "tailored" dentalcements with certain optimal properties, especiallybiocompatibility, durability and adhesiveness. One approachfor improving the overall properties of dental cementsinvolves developing hybrid cement-composites or resin-modified cements. Of special interest are the glass ionomercements, a class of relatively high modulus cements whichadhere to enamel and, to a lesser extent, to dentin. Withminimal surface preparation, adhesive strength is higher to

enamel than dentin (presumably because of the lower mineralcontent of the latter)

,but the system is relatively weak in

either case because of the low tensile and flexural strengthinherent in this type of material. Glass ionomer cementsalso display a susceptibility to excessive hydration duringthe early stages of their setting reaction which results inan inferior cement. In addition, the hardened cement has a

propensity to dehydration, brittle fracture, and erosionunder acidic conditions.

Objective

To enhance the durability of dental cements by improvingtheir hydrolytic stability, especially under acidic oralconditions (e.g., in plaque coated areas), and by moderatingtheir brittle nature while increasing their tensile flexuralstrengths

.

Phase I Formulation and Evaluation of New Cements

(a) Resin-and Dolvmer-modified glass ionomer cements.

Our previous investigations demonstrated the feasibilityof developing hybrid dental cement-composites (HCC's)from polyelectrolyte based cements (e.g. glass ionomercements) and water soluble or compatible vinyl monomerand initiator systems (Fig. 3) These resin-modifiedglass ionomer cements exhibited a significantimprovement in their resistance to brittle failure and

43

usually displayed higher diametral tensile strengthsthan conventional glass ionomer cements. In this studywe explored the feasibility of preparing polymer-modified glass ionomer cements from a water hardenableglass ionomer cement using aqueous solutions of variouspolymers (e.g. polyethylene oxide, polyvinyl alcohol,gelatin, etc.). This type of modified glass ionomercement does not require the use of compatible resin andinitiator systems, although these may be used along withthe water soluble or compatible polymers. The use of

the aqueous polymer solutions facilitated the mixing of

the blended powder component (polyacid and ion- leachableglass) of the glass ionomer cement with water andyielded toughened cements with excellent diametraltensile strengths (Table 4) . An abstract based on this

work has been submitted for the 1989 AADR meeting.

(b) Formation of Hvdroxvanatite in Hydrogels fromTetracalcium Phosnhate/Dicalcium Phosphate Mixtures(New^

Due to the higher priority assigned to part (a) of PhaseI no additional work was performed on this projectduring this period.

44

CH CH-2 I

CaAlFSlO HO3 --i—

POUACID 10N-J^A|gABLE

BLENDED POWDER

-

Ca,Al PolyMllHydPogtl

4

CiAlFSlO3

"lONOMER CEMENT"

nCH

CH

i-

CM

I

'

CH

1^

OH

MEMA

HOOM AA

2(Cu

M 02

HEMA - 2-MYDnOXYETMIL MEniACRILATE

AA - ASCORBIC ACID

HOOM - HYDROCEN PEROXIDE

2Cu - SOLUBLE CUPRIC SfZT

0

L

CM

CM

OHn

POLY(MEMA)

CaWFSlO3

(AI Cu )

HEMA/H 02

HOOM

Ca,Al POLYSALT

HYDROGEL

CaAlFSlO3

POLY (HEMA)

MODIFIED BLENDED P3WDER

"HYBRID CEMENT

COMPOSITE"

AA - ASCORBIC ACID

HOOM - HYDROGEN PEROXIDE

2Cu - SOLUBLE CUPRIC SALT

(c)

Poly mer Izat ion mechaniama for (a) conventional water harcienable

glaaa ionomer cement, (b) aqueous free radical polymerization of

water aoluble monomer, and (c) hybrid cement composite (HCC).

45

TABLE 4

DIAMETRAL TENSILE STRENGTH OF A POLYMER MODIFIEDGLASS -lONOMER CEMENT rCHF.MFIL II)

POWDER*POLYMER % b.w. in H2 O Liquid

PEO-8,000 6.4 8

PEO-8,000 10.1 8PEO-100,000 1.0 8PEO-100,000 2.0 9PEG-20,000 2.3 7PEG-20,000 2.3 8PEG-20,000 9.9 7PEG-20,000 9.9 8GELATIN 5.0 7GELATIN 20.0 6.7

^CHEMFIL II POWDER

DTS(S.D.

)

in MPa

27.8(3.6)28.9(3.1)31.0(4.7)31.6(3.4)32.1(3.4)33.9(3.3)26.2(1.9)29.0(0.4)19.7(2.3)21.9(3.3)24.4(1.3)7d DTS

46

III. Development of Improved Interfacial Bonding Systems andFillers for Composites and Cements

Overview (Filler)

On a weight basis, the major component of dental compositesis usually the reinforcing filler (e.g., 50-86%). For manyproperties of the composite, the volume percent of thedispersed phase is a more significant parameter. Thereinforcing filler performs many functions in a compositesuch as stiffening the lower modulus resin binder, therebyincreasing mechanical properties, enhancing dimensionalstability, moderating the exotherm of polymerization and themismatch in the thermal expansion of the organic matrix andtooth structure, reducing water sorption and polymerizationshrinkage, and aiding in matching tooth appearance. By usingglass or ceramic fillers that have refractive indicesapproximating those of the matrix, they can be used to formtranslucent fillings that match the normal translucency oftooth structure. The selective inclusion of compounds withelements of high atomic number (e.g., barium, strontium,lanthanum, zinc, zirconium, titanium, etc.) in thepreparation of glass fillers yields esthetic composites witha degree of radiopacity.

A variety of types, shapes, and sizes of fillers have beenused in dental composites, e.g., quartz, fused silica,borosilicate and aluminosilicate glasses, silicon nitride,calcium silicate, calcium phosphates, aluminum oxide, metals,etc. In addition, submicron fillers such as precipitated or

pyrogenic silicas (0.14-0.007 fim) averaging 0.04 /xm in sizehave been used in microfilled composites. The high surfacearea of this type of filler makes it difficult to achievehigh filler loadings by weight in this type of composite,e.g., 50 weight percent is usually the maximum. ' To enhancetheir miscibility and dispersion in resin systems, smallorganic-rich, composite macrofillers are made frompulverized, prepolymerized composites derived from the

silanized, microfine fillers and the same or similar monomersystems. Composites formulated with these prepolymerizedcomposite fillers also are termed microfilled composites.

Compared to conventional composites with their larger fillersizes (0.7 to 100 fim, but usually 2-50 fim.) ,

microfilledcomposites have smoother, more easily polished surfaces whichmay reduce the adherence of plaque and stains. On the otherhand, they have lower moduli and tensile strength, exhibitmore creep, and have higher water uptake, thermal expansionand polymerization shrinkage than conventional or hybridcomposites

.

47

Hybrid composites, which incorporate, major quantities of thesmaller sized macrofiilers along with small amounts ofmicrofillers, achieve almost as smooth a surface texture as

the microfilled composites without compromising (oftenactually improving) other properties. Some of the newerhybrid composites have a multimodal dispersed phaseconsisting of different types, shapes and sizes of fillers[47-51].

An innovative approach to enhance the interfacial bonding ofthe inorganic and organic phases of the composite is throughthe use of "semiporous” glass fillers obtained by selectivelyacid etching the more soluble phase of glass particles havingtwo interconnected vitreous phases [47,48]. Properly done,

this results in a glass filler having superficial surfaceporosity into which the resin can flow and mechanicallyinterlock on polymerization, thus complementing the usualbonding through silane coupling agents.

The search for dental composites of superior wear resistancefor use in stress -bearing applications has spurred researchinto new types of stable fillers of sizes and shapesconductive to optimal packing efficiency. New techniques to

significantly increase filler loadings have appeared in

[49,50].

Overview (Interfacial Bonding Systems)

Although it is only a minor component of resin-based dentalrestorative materials, an interfacial bonding agent exerts a

profound effect on the durability of composites. The qualityof the interfacial bonding phase existing between the

polymeric matrix and the dispersed phase exerts a significanteffect on the ultimate properties and the clinicalperformance of dental composites. Even composites preparedfrom the best of resin binders and reinforcing fillers willbe deficient in durability if water and other contaminantspenetrate and disrupt the interfacial bonding phase.

Bifunctionai coupling agents such as organofunctionalsilanes, titanates

,zirconates, etc., are used in composites

to promote adhesion between mineral fillers and organic resinbinders [52]. Alkoxysilanes having terminal vinyl groupshave been the most widely used type of coupling agent fordental composites. Initially, a vinyltrialkoxysilane wasused but it was later found that 3-

methacryloxypropyltrimethoxysilane was more effective withmethacrylates [51,53].

Alkoxysilanes can react with surface moisture, usuallypresent at least as a monolayer on mineral surfaces, to

generate silanol groups which can strongly hydrogen bond to

48

hydroxylated surfaces. In addition, silanol groups can reactchemically with surface hydroxyl groups of the filler viacovalent bond formation. There is some direct (e.g.,

spectroscopic) evidence that suggests that these kinds ofreactions do occur between silane coupling agents and manyt3rpes of mineral fillers used to reinforce composites [52] .

Organofunctional silanes can be visualized as reacting byboth hydrogen bonding and/or covalent attachment to mineralfillers by virtue of their silanol or derivative groups andby copoi3rmerization with the resin system via their terminalvinyl groups. Indirect evidence for this interfacial bondingis provided by the observed enhancement in mechanicalstrength and resistance to water and other chemicals ofsilanized composites [51-53].

The effectiveness of coupling agents in a composite dependson a number of factors: (1) the nature of the resin binderand filler, (2) the structure and chemical reactivity of thecoupling agent (3) the amount used, and (4) the mode ofapplication.

Objective

To develop filler and interfacial bonding systems of enhanceddurability applicable to dental composites and cements(filler portion of project is new).

PROGRESS REPORT

Phase I Development of Improved Interfacial Bonding Phasesfor Composites and Cements

This study has as its goal the elucidation of the natureof the interfacial phase that forms between the fillerand resin phases of dental composites. Previous studiesof composites prepared using a conventional radiopaqueglass filler (~45/im) demonstrated the significantlygreater diametral tensile strengths (DTS) and transversestrengths (TS) of composites prepared with silanized (3-

methacryloxypropyltrimethoxysilane,

MAPS) versus thosecontaining an equal amount of unsilanized filler (53) .

A similar study is in progress using a microfine glassfiller (Degussa OX 50)

.

Silanization required as

expected a larger quantity of MAPS (-7% b.w. versus 0.5

b.w. for the conventional filler). For a visible lightactivated ethoxylated bisphenoi A dimethacrylate resin(Esschem Co.) containing 50% b.w. of the silanized andunsilanized microfine glass filler, 24 hour DTS valuesof the respective silanized and unsilanized compositeswere 38 and 34 MPa, respectively. Apparently, at leastas regards this mechanical property, there seems to be

less of a reinforcing effect from silanized microfine

49

glass compared to that of silanized siliceousmacrofillers . The same ethoxyiated bisphenol Adimethacrylate resin gave with 80-82% b.w. of theradiopaque macrofiller (silanized versus unsilanized)results similar to those we reported previously for the

BIS-GMA/TEGDMA based composites; that is, the DTS valueof the composites containing silanized macrofiller werealmost twice that of similar composites formulated withunsilanized glass.

In a cooperative study with Drs. Katz and Singh ofRensselaer Polytechnic Institute, the elastic propertiesof composites prepared with a unsilanized and silanizedradiopaque silica macrofiller (45/im) were evaluatedusing ultrasonic propagation techniques. Under drystorage conditions the composites had similar elasticproperties [55].

In a related study using it was demonstrated the MAPSand related silane coupling agents were ineffective inconferring reinforcing properties (as measured by DTS)on various calcium phosphate fillers. Infrared analysisindicated that after washing with the solvent used in

the silanization procedure, silane derived products werepresent on the calcium phosphate minerals. However, the

interfacial bond appears to be weaker than that obtainedwith siliceous fillers. Preliminary results using a

zirconate coupling agent, zirconium (IV) dimethacrylate,as a means of surface modifying calcium phosphatefillers appear to be more effective. [56,58].

In another study we have demonstrated the vulnerabilityof hydrophilic composites to the adverse effects ofwater. In Table 5 is shown the composition of anextremely hydrophilic resin system containing almost 50%

HEMA (2 -hydroxyethyl methacrylate) and chemicallyactivated with N,N-dihydroxyethyi-p- toiuidine (DHEPT)

.

Composites prepared from this resin and a conventionallysilanized radiopaque macrofiller (45;im) using a

powder/liquid ratio of five exhibited excellent DTS (54

MPa) after one day storage in distilled water at 37 °C.

However, after two weeks of aqueous storage at 37®C, DTS

had fallen to 44 MPa. This result probably reflectsplasticization of the matrix by the sorption of water,

which would in turn reduce the glass transitiontemperature of the composite. Degradation of the

interfacial silane bond also may be affected, especiallyafter more prolonged storage.

A water -saturated resin formulation based on that shownin Table 5 was prepared by the addition of 10.2% b.w. of

distilled water. A single phase resin system resulted

50

which when mixed with 5 parts b.w. of glass yielded a

composite with a DTS of 46 MPa after one day storage inwater at 37 “C, similar to the value obtained from the

"dry" hydrophilic composite after 14 days storage.Presumably, the lower DTS value is again mainly due to

the plasticizing effect of water on the composite,although deterioration of the interfacial phase also maybe occurring.

Phase II. Development of Improved Filler Systems forComposites and Cements.

Filler Systems Based on Silica

The effect of filler content on the characteristicstrength (S^) and Weibull modulus (m) has been evaluatedfor a BIS-GMA based composite using Diametral TensileStrength (DTS) values. The resin consisted of 69.3%b.w. BIS-GMA and 29.7% b.w. TEGDMA activated with a

visible light initiator system comprising 0.2% b.w.

comphorquinone plus 0.8% b.w. ethyl 4-

dimethylaminobenzoate . A commercial visible lightsource (Prismalite, L.D. Caulk Co.) was used to effectpolymerization of the composite. The particulatemacrofiller (45 pim) was a barium oxide containing silicasilanized with 0.5% b.w. MAPS. On a weight/weight basisthe filler/resin ratio (F/R) was varied from 1.5 to 5.

For each F/R a total of 60 DTS composite specimens wereprepared, stored in distilled water for two weeks at37 "C, and tested at a crosshead speed of 10 mm/min on a

universal testing machine. Cylindrical specimens ofapproximately 6mm diameter x 3 mm height were used.Maximum likelihood estimates for m and S^ weredetermined by Newton-Raphson iteration as shown below:

Filler/Resin Ratio (wt/wt)

:

F/R 1.5 3 4 5

Weibull modulus m 10.7 10.1 11.8 10.0Characteristic Strength (MPa) S^ 44.7 44.6 48.8 52.4

The Weibull modulus appears constant for all F/R at the

90% confidence level. The characteristics strengths forF/R > 3 are statistically different at the 90%

confidence level, using tables by Thoman, Bain andAntle. It appears that the Weibull modulus m of the

resin governs the fracture strength distribution whereasthe filler content affects the fracture strength (Fig.

4).

51

TABLE 5

EFFECT QF lUQ ON TllE STREIIGTll OF RESIN BASED_CQnrDSITE 5

FORMULATION AND EVALUATION OF A HIGHLY HYDROPHILICRESIN BASED COMPOSITES

COMPOSITION OF RESIN COMPOSITION OF POWDER

COMPONENT WT%BIS-GMA 37.35

TEGDMA 12.45HEMA 49.80DHEPT 0.40

SILANIZED BaO-SlOj COATEDWITH 1% b.v. BPO

POWDER/LIQUID RATIO = 5

DIAllETRAL TENSILE STRENGTH (DTS) AFTER WATER imERSION AT 37*C

TIME OF IMMERSION IN H^O DTS (STD DEV) IN MPa

1 DAY14 DAYS

54.2 (4.2)

43.8 (1.4)

1 DAY USING RESIN WITH 10.2% b.w. HjO 45.6(2.0)

52

Filler Systems Based on Calcium Metaphosphates

This project represents a collaborative effort with B.

Fowler of NIDR and S. Venz of Caulk/Dentsply

.

Orthophosphate minerals such as tribasic calciumphosphate, various synthetic apatites and hydroxyapatitehave been considered as fillers for resin based dentalcomposites [56-58]. Because of their high refractiveindices (e.g. 1.62 or greater) these types oforthophosphate minerals create a serious opticalmismatch when they are used as fillers for currentdental resin systems. Because of their opaqueappearances, these composites lack the esthetics neededfor the restoration of anterior dentition. Also,

because of this optical mismatch between the resin andfiller phases, these composites are ill-suited forphotochemical polymerization. They can, however, beseif-cured by the usual methods of ambient, chemicalfree radical polymerization (e.g. benzoyl peroxide andtertiary aryl amines).

Calcium metaphosphates (CMP's) are a unique class ofphosphate minerals having oligomeric or polymericstructures (Fig. 5), [Ca(P03 ) 2 ]n, analogous to that ofsome calcium metasiiicates

,[CaSiOgj^^. The PO

3units

are structurally linked to form chains that have lengthsranging from about 20 to 10,000 units (n = 10 to 5,000)

[59] . Four different crystalline forms designated a,

P, 7 ,8 and a noncrystailine or vitreous form of calcium

metaphosphate are known. The approximate values of nare: 5,000 for the a and ^ forms; 200 to 600 for the 7and 8 forms; and 20 for the vitreous form [59].

Physical and Chemical Properties of Ca.(V0-^ )_2

Thermal Stability and Melting Point . The thermalstabilities of these polymorphs increase in the 8-'y-P-a

sequence [60-62] . The 5 and 7 forms convert into the

form at about 700"C and 500“C, respectively; the yS formexists up to 963®C and then melts at 977°C. The a formexists between 963“C and its melting point of 984°C.

Indices of Refraction . Values for the crystalline a,

7 and 8 forms range from 1.57 to 1.59 and that of the

vitreous form is 1.544 [61,63]. Thus unlike, many othercalcium phosphate fillers, CMP's have refractive indices(~ 1.54-1.59) that are closer to those of many dentalresin systems.

53

CUMULATIVE

PROBABILITY

OF

FAILURE

FOR

4

FILLER/RESIN

RATIOS.

00

o3dniivj

CD rt• •o o

JO 'aodd

CN

dnniAino

qd

54

DIAMETRAL

TENSILE

STRENGTH

(MPa)

Li. COO UJ

LU <^ %H" COQOcr CL

CO 1“LU

gGCLU

CL O

O—Q-—

o

I

01

Q.I

01

Q.I

01

Q.I

01

Q.I

01

%C8O0«CL-0

o—Q.-0

* ^O—COO

m

FIGURE

RENG

TO

pH of Water Slurry. A slurry of 0.2 mass % ^-Ca (P03)2in water has a pH of 4.5 ± 0.1; vitreous-CaCPOg )£ 0.5 to

5 mass % slurries have a pH of 3.5 ± 0.2.

Solubility in Water. The crystalline a, /3, 7 and 5

forms are very insoluble in water at low pH; the

vitreous form is more soluble than the crystalline forms

[61,64]. Preliminary solubility values in grams per 100

ml of water at 22‘’C are: ^-Ca(P03 ) 2 ,0.01 g at pH 4.5 ±

0.1; vitreous - Ca(P03 ) 2 ,about 0.2 g at pH of 3.6 ± 0.1

on dissolution for three days. The correspondingsolubilities of hydroxyapatite, the prototype of themineral in tooth and bone, are 0.08 g at pH of 4.5 and,

by extrapolation, about Ig at pH 3.6 [65].

Density . Calculated values in g per cm^ for the a, /3,

7 ,6 and vitreous forms of Ca(P03)2 are: 2.96, 2.92,

2.85, 2.94, and 2.72, respectively [63]. The measureddensity of the ^ form is 2.82

[63].

Hardness . ;6 -Ca(P03)2 and vitreous-Ca(P03 >2 have Knoop

hardness values in N per mm^ of 191 ± 25 and 288 ± 2,

respectively.

Preparation of Calcium Metaphosphates

The four crystalline and vitreous forms of Ca(P03)2 canbe prepared by appropriate heating of commerciallyavailable monocalcium phosphate monohydrate,Ca(H2 P04 ) 2 .H2 O, in the 150 to 1050“C range [60,61,64].Procedures for preparation of the lower temperatureforms, 7 and S of Ca(P03 ) 2 ,

are reported in the

literature [61,63].

B-Ca.(?0-^ Preparation . Heat Ca(H2PO^ )

2

.

H

2 0 at 150 to

180'’C to remove hydrate water and form anhydrousCa(H2 P04 ) 2 ;

further heat to 250 to 300°C to effectcondensation of Ca(H2 P04)2 groups with release of two

water molecules per formula unit to form [Ca(P03 ) 2 ]n

chain structures. The [Ca(P03 ) 2 ]n (formula givenwithout n hereafter) first formed on heating for a fewhours at 250'’C is noncrystalline. This can be convertedto crystalline

;9 -Ca(P03)2 by the heating temperature and

duration of heating in the 250 to 963 °C range. Theapproximate temperatures and times, respectively,required for near 100% conversion to crystalline /3-

Ca(P03)2 are: 250“C, 5 days; 500®C, 2 hours; 600°C, 1

hour; and 950 °C, less than 10 minutes.

56

Q!-Ca(P03 )2 Preparation . The a-Ca(P03)2 form existsbetween 963 “C and its melting point of 984'’C. HeatCa(P03)2 between 970 to 980®C for conversion to a-Ca(P03)2 [60,66].

Vitreous-CarPOj ^2 Preparation . Heat Ca(H2 P0 ^

>

2 .H2 O orCa(H2P04)2 or any of the Ca(P03)2 forms in a platinumcrucible at 1050“C to form a Ca(P03)2 melt; removecrucible containing melt at 1050®C and allow melt to

rapidly cool in air at ambient temperature to roomtemperature [63,64,66]

.

Preparative Conditions that Influence and/or Control theSize. Shape. Texture. and Porosity of CalciumMetaphosphate Solid Particles . The dehydration ofmonocalcium phosphate monohydrate (Reaction I) andcondensation of monocalcium phosphate (Reaction II) to

form calcium metaphosphate result in the evolution ofone and two molecules of H2 O, respectively;, the overallreaction is given by Reaction III.

I.

150“CCa(H2P0^ )2 .H2 O Ca(H2 P0^)2 + H2 O

II.

250*0

Ca(H2P0^)2 Ca ( PO 3 )

2

+ 2H2 O

III.

250*CCa(H2P0,)2.H20 - Ca(P03 )2 + 3H2 O

Rapid Dehydration and Condensation to Form ^-Ca('P03 JI2

If Ca(H2 P04, )2 .H2 O or Ca(H2 P04)2 is heated directly at

500 to 950®C at atmospheric pressure the suddenevolution of water from the solid during the dehydrationand/or condensation reactions results in very porousparticles .of

;9 -Ca(P03 ) 2 . For example, direct ignition

of Ca(H2 P04 )2 .H2 O at 900 to 950°C for 3 to 20 minutesforms irregularly shaped crystalline birefringentparticles of ^-Ca(P03)2 up to 200-300 urn in size thathave nximerous craters on the surfaces and internal poreswith diameters from about 1 to 5 urn. The degree of

porosity formed by heating directly at 900®C, e.g., 10

minutes, can be controlled and progressively reduced byfurther heating about 20°C below the melting point of

977“C for increasing time periods. The degree of

formation of pores in Ca(P03)2 particles prepared by the

dehydration and condensation reactions (I and II) are

affected by vapor pressure. Direct ignitions at 800°Cfor 15 minutes (from 25 to 800°C in about 5 minutes)under reduced pressure (26 Pa)

,atmospheric pressure

(10^ Pa), and under increasing water vapor pressure of

about 5 to 30 X 10^ Pa yielded porous to nonporous

57

particles. Porous particles were obtained at 26 Pa, 10^

Pa, and about 5 X 10^ Pa of water vapor pressure;nonporous smooth surface particles were obtained betweenabout 10 to 30 X 10^ Pa of water vapor pressure.Mixtures of both porous vitreous-CaCPOg >2 and ^-Ca(P03)2can be prepared by heating Ca(H2 P04 ) 2 .H2 O or Ca(H2 P0^)2at selected temperatures for specific times: directheating at 500 “C for 10 minutes mainly forms vitreous-Ca(P03 ) 2 ;

after 30 minutes at 500®C, the product is

about 25% vitreous and about 75% ^-Ca(P03 ) 2 ;and after

several hours at 500®C, it is essentially all /3-

Ca(P03 ) 2 . The solid products formed by heating directlyat 500®C are less porous than those heated directly at

900®C. In composites formulated from these fillers withcratered surfaces and pores, resin incorporation intothese craters and pores is expected to physicallyincrease adhesion between resin and filler and to

increase the composite strength. (Preliminary testsshowed composites formulated with 85 mass % porous /3-

Ca(P03)2 and 15 mass % polyethylene have about 10 %

higher diametral tensile strength than those fromnonporous controls)

.

Progressive Dehydration and Condensation to Form B-

Ca(.P03^2

If the dehydration and condensation reactions ofCa(H2 P0 ^

>

2 .H2 O are carried out progressively, e.g., (a)

150 ®C for several hours to effect dehydration, (b) 250 ®C

for several hours to effect condensation, and (c) thenheated at higher temperature to accelerate formation of

;5 -Ca(P03 ) 2 ,

500°C for about 2 hours or 900°C for about10 minutes, the particles are less porous have smoothersurfaces than those formed by direct heating between 500

and 900° C.

Formation of a-, 8- and 5 -Ca('P03 the MeltAccording to the P2 O 5

-2CaO.

P

2 O 5phase diagram [60], on

cooling a melt of Ca(P03)2 initially above 984°C, a-

Ca(P03)2 first crystallizes at 984°C and then convertsto

)9 -Ca(P03)2 at 963°C which is the stable form on

cooling to room temperature. On cooling melts ofCa(P03 ) 2 ,

initially at 1050‘’C, at about 5°C per minute,the melts supercooled about 80°C below the

crystallization temperature of a-Ca(P03)2 andcrystallization occurred rapidly between about 920 to

900°C. Under these cooling conditions along withquenching the sample to room temperature within about 10

to 30 minutes after crystallization occurred, the S

polymorph of Ca(P03)2 predominately formed. If these

first formed 6 -Ca(P03)2 crystals from the melt were

heated at 900 “C for several hours, they converted to the

stable polymorph expected for this temperature, fi-

58

Ca(P03 ) 2 * Formation of 5 -Ca(P03)2 from the melt hasapparently not been reported. Previous methods for 8 -

Ca(P03)2 preparation, from Ca(H2 P04)2 at about 300 to600*C were shown to be unreliable [61,62]. The groundparticles of 5 -Ca(P03)2 were tabular-like in contrast to

the fibrous-like particles of ground ^-Ca(P03 ) 2 .

Supercooled melts at 900‘’C that were seeded with a fewcrystals of

)9 -Ca(P03)2 rapidly solidified and formed 0 -

Ca(P03)2 crystals that were elongated and cleaved ongrinding into fibers. Initial nucleation of thesupercooled melts from point sources, a few

;6 -Ca(P03)2

seed crystals, resulted in radiating, directional,linear type growth. Rapid crystallization from theseeded supercooled melts promotes formation of longer ^-

Ca(P03)2 crystals than those formed by diffusion insolid-state reactions. The a-Ca(P03)2 form was notobtained from the melt under the above coolingconditions; seed crystals of a-Ca(P03)2 to nucleate it

in the melt were not available. Heating solidcompositions of Ca(P03)2 at 970 ± 2®C for sufficienttime is reported [64,66] to form a-Ca(P03 ) 2 . Since thea form exists in a narrow temperature range of 963 to

984'*C, accurate and precise temperature control is

necessary to prepare it.

Vitreous-Ca(P03 >2 Particle SizesThe solidified vitreous-Ca(P03 >2 prepared by rapidlycooling the melt is a clear, transparent glass -likematerials. The physical volume of the intact solidvitreous-Ca(P03 )2 is primarily limited only by the

volume of the vessel used to prepare the melt. Theparticle sizes can be varied by conventional grindingtechniques and sieving. A paper presented at the 1988

lADR meeting gave some preliminary results of using ^-

Ca(P03)2 and vitreous Ca(P03)2 in resin based composites

[67]. A patent application also has been submitted.

IV. Bonding of Low Surface Energy Resin Systems to ToothStructure (New Project)

Objective

To develop durable bonding systems for tooth structurecompatible with a variety of resin-based dental materialsespecially those of low surface energy.

Overview

Current resin-based dental composites do not adhere to toothstructure. However, the discovery of the acid etch technique

59

made it possible to bond resin-based dental materials to

enamel by a micromechanical interlocking mechanism [5,6].Surface microporosity is generated on enamel by a briefpretreatment with aqueous phosphoric acid or certain types oforganic acids e.g, pyruvic, citric, etc. The usual acid etchtechnique is generally ineffective and contraindicated foruse with dentin.

Adhesion to dentin has presented a more challenging problem.On a weight basis, dentin consists of 69% hydroxyapatite, 18%

organic matter (mainly collagen) and 13% water. An adhesivebonding or coupling agent for this substrate would mean lessinvasive cavity preparations with decreased loss of soundtooth structure and a reduction in microleakage with its

potential for secondary caries formation. Considerableeffort has been devoted to the development of coupling agentsthat can mediate bonding between dental resins and apatiticsubstrates [7-16]

.

Surface -active comonomers that can bond to apatiticsubstrates by chelation of surface Ca^'*' and/or othermultivalent cations have been made by the reaction of N-

substituted aryl glycines (e.g. N-phenylglycine (NPG) and N-

p- tolylglycine (NTG)]

with giycidyl methacrylate (GMA)

.

Other types of coupling agents are functional vinyl monomersthat have groups capable of reacting with collagen byspecific chemical reactions, e.g., esterification, urethane,urea or Schiff base formation. Another approach forchemically bonding to collagenous substrates involves graftpolymerization techniques using free radical initiation [7-

16].

Recently, three types of adhesion-promoting systems havedemonstrated rather strong adhesion to dentin. One systeminvolves pretreatment of dentin with an aqueous solution of

2-hydroxyethyl methacrylate (HEMA) and glutaraldehyde [14].

The second bonding system is based on the functional monomer,4-methacryloxyethyl trimellitic anhydride (4-META) and a

pretreatment of dentin with a ferric salt such as ferriccitrate [9,10]. The third bonding procedure utilizes a briefapplication of a cleanser, mordant aqueous solution of ferricor aluminum oxalate, a surface active agent (e.g. NPG, NTG)

or comonomer such as NPG -GMA or NTG -GMA and PMDM, the

diadduct of HEMA and pyromellitic anhydride [8,15,16]. The

latter two bonding systems display significant adhesion to

enamel as well as dentin.

60

PROGRESS REPORT

Phase I. Synthesis of Carboxylic Acid- Containing Monomers.

Further work was deferred on this project while waitingfor a more complete characterization of the diadduct oft-butylaminoethyl methacrylate (tBAEM) with 3, 3', 4,4'-

benzophenone tetracarboxylic dianhydride, BTDA, by HPLCand NMR analyses

.

Phase II. Aldehyde Based Bonding Agents

In its present version, the Gluma system (HEMA plusglutaraldehyde) for bonding composites to dentininvolves the prior use of a bonding agent. In a

cooperative study with E. Asmussen and R. Bowen a studywas undertaken to determine if pretreatment of thedentin with solutions of amino acids and theincorporation of camphorquinone and selected monomersinto Gluma would negate the need for a bonding agent.Using a solution of pyruvic acid and glycine fordentinal pretreatment

, and an optimized adhesive mixtureof glutaraldehyde, HEMA, BIS-GMA, camphorquinone andwater, tensile bond strengths to dentin of 14.5 MPa andto enamel of 23.3 MPa were obtained. A bond strength to

dentin of 13.4 MPa was obtained in a control experiment(Gluma & Bonding agent) . This new type of Gluma bondingsystem thus gave acceptable bond strengths without theprior application of enamel bonding agents. Amanuscript based on this work has been approved by WERBand submitted for publication.

References

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Dent. Res. 3̂ 360 (1956).

[lb] Bowen, R.L. Dental filling materials comprising vinyl silanetreated fused silica and a binder consisting of the reactionproducts of bisphenol and glycidyl methacrylate, U.S. Patent

3,066,012 (1962).

[2] Antonucci, J.M. Resin-based dental composites -An overview,

in Polymers in Medicine II eds

.

Chielini, Giusti, P.,

Migliaresi, C. and Nicolais, L. Plenum Publishing Corp. ,

NY,

NY, pp. 277-303 (1986).

[3] Wilson, A.D. The chemistry of dental cements. Chem. Soc.

Rev. 7 265 (1978)

.

61

[4

]Wilson, A.D.

, Prosser, H.J., and Fowls, D.R. Mechanism ofadhesion of polyelectroiyte cements to hydroxyapatite. J.

Dent. Res. 62 590 (1983)

.

[5] Buonocore, M.G. Simple method of increasing the adhesion ofacrylic filling materials to enamel surfaces. J. Dent. Res.34 849 (1965).

[6] Silverstone, L.M. The structure and characteristics of humandental enamel in biocompatibility of dental materials. Vol.

1, Smith, D.C. and Williams, D.F.

,

eds.

(RC Press Inc.) BocaRaton, FL, Chap. 2 (1982).

[7] Brauer, G.M. Adhesion and adhesives in scientific aspects ofdental materials. Von Fraunhofer, J.A., ed. Butterworths

,

London, Chap. 2 (1975).

[8] Bowen, R.L.

,

Cobb, E.N. and Rapson, J.E. Adhesive bonding ofvarious materials to hard tooth tissues. Improvement in bondstrengths to dentin, J. Dent. Res. ^ 1070 (1982).

[9] Yamauchi, J., Nakabayashi, N. and Masuhara, E. Adhesiveagents for hard tissue containing phosphoric acid monomers

,

ACS Polym Prepr. 20 594 (1979).

[10] Atsuta, M.,

Abell, A.K.

,

Turner, D.T.

,

Nakabayashi, N. andTakeyama, M. A new coupling agent for composite materials:4-methacryloxyethyl trimellitic anhydride, J. Biomed. Mater.Res. 16 619 (1982).

[11] Antonucci, J.M. Aldehyde methacrylates derived fromhydroxybenzaldehydes . J. Dent. Res. ^ 500 (1978).

[12] Antonucci, J.M., Brauer, G.M. and Termini, D.J. Isocyanateurethane methacrylates derived from hydroxyethylmethacrylate. J. Dent. Res. ^ 35 (1980).

[13] Asmussen, E. and Munksgaard, E.C. Bonding of restorativeresins to dentin by means of methacryloyl chloride andmethacryioyl-R- isocyanate

,Scand. J. Dent. Res, £1 153

(1983)

.

[14] Asmussen, E. and Munksgaard, E.C. Bonding strengths betweendentin and restorative resins mediated by mixtures of HEMAand glutaraldehyde . J. Dent. Res. ^ 1087 (1984).

[15] Bowen, R.L. and Cobb, E.N. A method of bonding to dentin andenamel. JADA 107 734 (1983).

[16] Bowen, R.L.

,

Cobb, E.N. and Misra, D.N. Adhesive bonding bysurface initiation of polymerization. Ind. Eng. Chem. Prod.

Res. Dev. 23 78 (1984).

62

[17] Wu, W.,and McKinney, J.E. Influence of chemical on wear of

dental composites. J. Dent. Res. 6̂ 1180 (1982).

[18] Wu, W. and Debelius, E.E. Environmental resistance of dentalrestorative materials. J. Dent. Res. £3 199, Abstr. 255(1984).

[19] Wu, W.,Toth, E.E., Moffa, J.F. and Ellison, J.A. Subsurface

damage of in vivo worn dental composite restorations. J.

Dent. Res. 63 675 (1984)

.

[20] Soderhoim, K.J. Degradation of glass filler in experimentalcomposites. J. Dent. Res. ^ 1867 (1981).

[21] Ibid. Leaking of fillers in dental composites. J. Dent.

Res. 62 126 (1983).

[22] Ibid. Hydrolytic degradation of dental composites. J. Dent.

Res. 63 1248 (1984)

.

[23] McKinney, J.E. Influence of acids on wear of compositerestoratives. J. Dent. Res. £3 199 Abstr 256 (1984).

[24] Antonucci, J.M., Venz,

S., Stansbury, J.W. and Dudderar, D.J.

Low surface energy dental composites from a poiyfluorinatedprepoiymer multifunctional methacrylate. Proceedings of the

1st Medical Plastic Conference of the Society of the PlasticsIndustry, Inc., New Brunswick, NJ (1983).

[25] Kuo, J.S., Antonucci, J.M. and Wu, W. Evaluation ofsiloxane- containing dental composites. J. Dent. Res. ^ 178,

Abst. 30 (1985).

[26] McKinney, J.E. and Wu, W. Effect of cure on hardness andwear of three commercial dental composites. J. Dent. Res. £2285, Abst. 1047 (1983).

[27] Antonucci, J.M., Stansbury, J.W., and Dudderar, D.J. Dentalresin and initiator systems based on polythiols. J. Dent.

Res. 61 270, Abst. 824 (1982).

[28] Venz, S. and Antonucci, J.M. Effect of a polythiol on the

degree of polymerization of resins. J. Dent. Res. ^ 199,

Abst. 257 (1984).

[29] de Rijk, W.G.

,

Conner, M.L. Jennings, K.A. and Wu. W. The invivo wear resistance of dental composites with enhancedpolymerization. J. Dent. Res. ^ 286, Abst. 951 (1983).

63

[30] Antonucci, J.M. New monomers for use in dentistry. Biomed.and Dental Applications of Poi3Tners, eds

. ,Gebelein, C.G. and

Koblitz, F.F. Plenum Press, NY, NY, 357-371 (1981).

[31] Antonucci, J.M., Stansbury, J.W., and Venz,

S. Synthesis ofa polyfluorinated prepol3nner multifunctional urethanemethacrylate. J. Dent. Res. ^ 209, Abst. 308 (1985).

[32] Bowen, R.L. and Antonucci, J.M. Dimethacrylate monomers ofaromatic diethers, J. Dent. Res. ^ 594-604, (1975)

[33] Anzai, M. and Ohashi, M. Studies on the reaction product ofhexachlorocyclotriphosphazene and 2 -hydroxyethyl methacrylateand on the physical properties of the polymer, J. Nihon Sch.Dent. 26 109 (1984).

[34] Ibid. Studies on composite resins with hexa(methacryloyl-ethylenedioxy) cyclotriphosphazene used as a monomer. J.

Nihon Sch. Dent. 26 238 (1984).

[35] Venz, S. and Antonucci, J.M. Degree of cure andpolymerization shrinkage of photoactivated resin-basedmaterials. J. Dent. Res. ^ 229, Abst. 488 (1985).

[36] Akkapeddi, M.K. (1979): Poly (a -methylene- 7 -butyro lactone )

;

Synthesis, Configurational Structure, and Properties,Macromolecules 12: 546-551.

[37] Akkapeddi, M.K. (1979): The free Radical Copolymerization ofa-Methylene- 7 -butyrolactone ,

Polymer 20: 1215-1216.

[38] Ruyter, I.E. and Gyorosi, P.P. An infrared spectroscopicstudy of sealants. Scand. J. Dent. Res. ^ (1976).

[39] Ruyter, I.E. and Swendsen, S.A. Remaining methacrylategroups in composite restorative materials. Acta, OdontolScand 36 75 (1978) .

[40] Ferracane, J.L. and Greener, E.H. Fourier transform infraredanalysis of degree of polymerization in unfilled resins:methods comparison. J. Dent. Res. ^ 1093 (1984).

[41] Asmussen, E. Restorative resins: hardness and strength vs

quantity of remaining double bonds. Scand. J. Dent. Res. 90

484-489 (1982).

[42] Cohen, S.g, Parola, A. and Parsons, G.H. Photo reduction byamines. Chem Rev. T3 141J61 (1933).

[43] Brauer, G.M. and Argentar, H. Initiator-accelerator systems

64

for dental resins, in initiation of polymerization, ED. F.E.Bailey, Jr. ACS, pp. 359-371 (1983).

[44] Otsu, T. ,Sato, T. and Ko, M. Polymerization of methyl

methacrylate with dimethylbenzylaniliniiom chloride. J. Poly.Sci. Part A-17 3329-3336 (1969).

[45] McLean, J.W., Powis,

D.R., Prosser, H.J. and Wilson, A.D.

The use of glass ionomer cements in bonding composite resinsto dentine. Br. Dent. J. 158 : 410-414 (1985).

[46] Brown, W. and Chow, L. U.S. Patent 4,418,430 (1985).

[47] Bowen, R.L. and Reed, L.E. Semiporous reinforcing fillersfor composite resins. I Preparation and provisional glassformulation. J. Dent. Res. ^ 738 (1976).

[48] Bowen, R.L. and Reed, L.E. Semiporous reinforcing fillersfor composite resins. II. Heat treatments and etchingcharacteristics. ibid 748 (1976).

[49] Cross, M.,

Douglas, W.H. and Fields, R.P. The relationshipbetween filler loading and particle size distribution incomposite resin technology. J. Dent. Res. ^ 850 (1983).

[50] Nemcek, J., Roberts, T.A.,

and Sherliker, F.R. U.S. Patent4,374,937 (1983).

[51] Bowen, R.L. Effect of particle shape and size distribution in

a reinforced polymer. JADA ^ 481 (1964).

[52] Pludderman, E.F. in Silane coupling agents. Plenum Press, NY(1982) .

[53] Venz,

S. and Antonucci, J.M. Silanization and modification offillers for dental composites. J. Dent. Res. ^ Abst. No.

191 (1986).

[54] Antonucci, J.M., Stansbury, J.W. and Venz, S. Synthesis of

silyl ether derivatives of BIS-GMA J. Dent. Res. ^ ABST. No.

451 (1986).

[55] Singh, S., Katz, J.L, Antonucci, J.M., Penn, R.W. and Tesk,

J.A. The elastic properties of glass reinforced dentalcomposites. J. of Non-Crystalline Solids 102 . 112-116

(1988) .

[56] Misra, D.N. Adsorption of zirconyi salts and their acids on

hydroxyapatite: use of salts as coupling agents in dentalpolymer composites, J. Dent. Res. 1405-1408 (1985)

65

[57] Antonucci, J.M., Misra, D.N. and Peckoo, R.J. Theaccelerative and adhesive bonding capabilities of surfaceactive amine accelerators. J. Dent. Res. ^ 1332-1342(1981)

.

[58] Rawls, H.R., Legeros, R.Z. and Zimmerman, B.F. A radiopaque

composite restorative using an apatite filler. J. Dent. Res.64 ABST No. 307 (1985)

.

[59] Ohashi, S. and Van Wazer, J.R. Structure and properties ofthe condensed phosphates. XIV. Calcium polyphosphates, J. Am.Chem. il 830-832 (1959).

[60] Hill, W.L.

,

Faust, G.T.

,

and Reynolds, D.S. The binary systemP2 05 -2Ca0.P2 05 . Part 1, Am. J. Sci. 2^, 457-477 (1944).

[61] Hill, W.L.

,

Hendricks, S.B., Fox, E.J., and Cady, J.G. Acidpyro-and metaphosphates produced by thermal decomposition ofmonocalcium phosphate, Ind. Eng. Chem. 39, 1667-1672 (1947).

[62] McIntosh, A.O. and Jablonski, W.L. X-ray diffraction powderpatterns of the calcium phosphates. Anal. Chem. 28. 1424-1427(1956).

[63] Lehr, J.R., Brown, E.H.

,

Frazier, A.W.

,

Smith, J.P. andThrasher, R.D. Crystallographic properties of fertilizercompounds. Chemical Engineering Bulletin No. 6, TennesseeValley Authority, Muscle Shoals, Alabama (1967).

[64] Hill, W.L.

,

Reynolds, D.S., Hendricks, S.B., and Jacob, K.D.Nutritive evaluation of defluorinated phosphates and otherphosphorus supplements. Assoc. of Official AgriculturalChemists 28 105-118. (1945).

[65] Fowler, B.O. and Kuroda, S. Changes in heated and in laser-irradiated human tooth enamel and their probable effects onsolubility, Calcif, Tissue Int. ^ 197-208 (1986).

[66] Hill, W.L., Faust, G.T. and Reynolds, D.S. The binary systemP2O5 -2Ca0.P2 05 . Part II, Am. J. Sci. 2^ 542-562 (1944).

[67] Antonucci, J.M., Fowler, B.O. and Venz,

S. Filler systemsbased on calcium metaphosphates. J. Dent. Res. ^ Abstr. No.

869 (1988).

Invited Talks

Antonucci, J.M., Stansbury, J.W. and Venz, S. Synthesis andproperties of a polyfluorinated prepolymer multifunctionalurethane methacrylate. American Chemical Society Symposium on

Progress in Biomedical Polymers. Los Angeles, CA Sept. 27, 1988.

66

Antonucci, J.M., Venz, S., McKinney, J.E. and Stansbury, J.W. Low

surface energy polymeric dental composites. American ChemicalSociety, Mid-Atlantic Regional Meeting, Symposium on HighPerformance Polymers, Lancaster, PA, May 26, 1988.

Antonucci, J.M. Hybrid glass - ionomer composite cements. TheSociety of Dental Materials of New York City, New York, New York,Jan. 7, 1988.

Publications

Antonucci, J.M. Toughened glass- ionomer cements. Trends andTechniques 5 (3) p. 4, 1988.

Antonucci, J.M., Stansbury, J.W., and Venz, S. Synthesis andproperties of a polyfluorinated prepolymer multifunctionalurethane methacrylate. Polym. Mat. Sci. Eng.

,Proceedings of the

ACS Division of Polymeric Materials, Vol. 59, 388-396, 1988.

Singh, Surenda, Katz, J.L., Antonucci, J.M., Penn, R.W.,and Tesk,

J.A. The elastic properties of glass reinforced dentalcomposites. Journal of Non- Crystalline Solids 102 (1988) p. 112-

116.

Manuscripts accented for publication or under review bv WERE

Kuo, J.S., Antonucci, J.M., Wu, W.L. and Venz, S. Evaluation ofsiloxane- containing dental composites. J. Dent. Res. in press.

Asmussen, E. Antonucci, J.M., and Bowen, R.L. Adhesion to dentinby means of Gluma Resin, Submitted for publication.

Contributed Talks

Antonucci, J.M., Fowler, B.O. and Venz, S. Filler systems basedon calcium metaphosphates. lADR, March, 1988, J. Dent. Res. 67 .

Abstr No. 869.

Venz, S., and Antonucci, J.M. Physical and chemicalcharacteristics of dual-cured dental composites. lADR, March,

1988, J. Dent. Res. 67_ Abstr No. 901.

Katz, J.L., Singh, S., Antonucci, J.M., Penn, R.W. and Tesk, J.A.

The elastic properties of polymeric-glass dental composites.lADR, March 1988, J. Dent. Res. ^ Abstr No. 856.

McKinney, J.E., Antonucci, J.M. and Venz, S. In vitro performanceof a dual-cured experimental composite containing a hydrophobic,flexible polymer matrix. lADR, March 1988, J. Dent. Res. ^ AbstrNo. 1188.

67

Sugawara, A., Antonucci, J.M., Takagi, S. and Chow, L.G. Formationof hydroxyapatite in hydrogels from tetracalciumphosphate/dicalcium phosphate mixtures. lADR, March 1988, J.

Dent. Res. 67. Abstr No. 2166.

68

II. WEAR RESISTANCE AND DURABILITY ASSESSMENT OF DENTAL COMPOSITERESTORATIVE MATERIALS

Overview

This part of the annual report includes descriptions and resultsof experimental procedures used by the NBS Dental and MedicalMaterials Group to evaluate the performance of dental compositerestorative materials. The methods employ both in vivo and invitro wear testing, microhardness, viscoelastic measurements, andmicrodefect analyses. The objectives are to use appropriatetechniques to define and delineate fundamental wear mechanismsapplicable to the in vivo wear of these materials. The resultsfrom these studies facilitate the development of improvedprototype composite restoratives here and at other laboratories,and, in some cases, to assess the extent of wear on the opposingenamel when the restorations are placed in stress -bearingocclusion (Class I and II) . The results from these studiesfacilitate the development of improved prototype compositerestoratives here and at other laboratories.

This task is divided into the following sections:

A. Wear and Durability Assessments of Composite Restoratives

Phase I. Recent Commercial Composites Which Appear Promisingfor Posterior Occlusal Application, (not active this period)

.

Phase II. Experimental Composites with Flexible Polymers.

Phase III. Glass -lonomer Cements: Conventional, Metal-Filled, and Modified Experimental.

Phase IV. Susceptibility of Commercial Dental Composites to

Topical Fluoride Gels.

Phase V. Experimental Studies on Experimental CompositesEmploying Apatite Fillers (not active this period)

.

B. Assessment of Wear of Human Enamel Against Dental RestorativeCounterfaces

.

Phase I. Assessment of Wear of Human Enamel Against a

Conventional Porcelain and a New Castable-CeramicCounterface

.

Phase II. Wear of Hydroxyapatic Against Dental-AlloyCounterfaces, (new task).

C. Glass-Transition Temperature (Tg

)

of Matrix Polymers (not

active this period)

69

D. Wear Instrumentation

Phase I. Methodology for Improved Wear Apparatus.

Phase II. Selection, Acquisition, and Assembly of ElectronicComponents

.

Phase III. Completion of Apparatus, Including Design andAssembly of Mainframe, Drive Units, and TransducerComponents

.

A. Wear and Durability Assessments of Composite Restoratives

Background

Clinical data and microdefect analyses on in vivo worn restorationbiopsies revealed that both mechanical wear and the intraoralenvironment play an important role in the degradation of compositerestorations. For this reason, we believe that a relevant andaccelerated wear test should, in some way, include the influenceof the intraoral environment. Accordingly, the wear testspecimens were preconditioned in appropriate liquids prior to

testing. Since dental composites comprise two major components,poi3niier matrix and inorganic reinforcing filler, appropriatefluids were selected to evaluate the corresponding degradationmechanisms separately.

Our earlier work involved microhardness and wear tests, along withcertain other tests, on commercial composite specimens in order to

delineate their degradation mechanisms. In order to studypotential problems with the matrix, the composite specimens werepreconditioned (usually for one week) in selected organic solventsfor which the solubility parameter was varied over the rangeapplicable to liquid- food ingredients. When the solubilityparameter of the solvent approximated that of the matrix resin,softening was observed by decreases in microhardness, whichusually corresponded to enhanced wear [1,2]. The influence of

preconditioning diminished during the course of wear as the wear-track depth into the specimen increased.

One serious limitation with dental polymers is that the curingprocess terminates at a low degree of conversion at the in situtemperature (37 °C). With the free-radical activated processes,the increase in viscosity during polymerization inhibits moleculardiffusion which, in turn, prevents the proximation of reactivespecies, which, in the meantime, are used up by oxidation. We

have found that increasing the degree of cure decreased the extentof softening and improved the wear resistance of the specimenswhich would otherwise be damaged from preconditioning [3,4]. The

degree of cure was enhanced by simply elevating the cure

temperature

.

70

Since a sufficient elevation in temperature would be impracticablefor in situ placed restorations, a more difficult andsophisticated approach involving changes in chemical structure is

one of our current activities as explained later in this chapterand Part I, Section F. It has to be remembered, however, thatincreasing the degree of cure may commensurate ly increase thepolymerization shrinkage, which may lead to a greater loss ofmargin integrity and corresponding leakage in application.

Accordingly, there are three important criteria to be satisfiedfor an appropriate matrix resin. These are (1) low solubilityparameter (below the range of liquid food ingredients ) , (2) a highdegree of cure (at in situ temperature) to further limit diffusionof liquids and subsequent swelling, and (3) low polymerizationshrinkage to maintain marginal integrity. The first experimentalcomposite used in this study was a hydrophobic, flexible resincomprising 70 wt % poiy(fluorourethane methacrylate) (PFUMA) and a

siloxane diluent bis (methacryloxypropyl) tetramethyldisiloxane(BIS-MPTMS). These were mixed with a silanized fused-quartzfiller. (For further details of structure and composition, seePart I, Section F of this report.) The polymerization shrinkagewas reduced by using the prepoiymer of PFUMA having about 10

repeat units which would correspond to molecular weight of 12,060g/mol.

The solubility parameter for the resin approximated that of pureethanol (2 . 6x10^ J ^/2 Accordingly, with the microhardnessand wear tests, ethanol and water were used as preconditioningfluids (for the latter no effect is expected)

.

From changes in hardness, it was apparent that considerable matrixsoftening resulted from the penetration of ethanol. On the otherhand, the wear was essentially unaffected. This result is

attributed to the high degree of crosslinking which, in this case,

permits some diffusion, but very limited dissolution from organicsolvents

.

Although the wear of the PFUMA/BIS -MPTMS system appeared to beunaffected by ethanol preconditioning, the absolute hardness wasless and the wear much larger [5] than that observed for the moreconventional resin-based composites. For this reason we haveinitiated a study involving composites with resins of a higherfluorine content for which more flexibility is expected, thus a

higher degree of cure. The resin contains a 70/30 mixture ofpolyfluoromethacryiate and hydrocarbon diluent monomers along witha photoinitiater

,a camphorquinone/tertiary amine, for visible-

light activation. The polymerization shrinkage was reduced byemploying a prepolymer of PFMA with 10 repeat units correspondingto a molecular weight of 10,320 g/mol. The experimental compositecontained 69 wt.% fine (2-4 /xm) silanized fused quartz. Both the

environmental resistance, as determined by Knoop Hardness Number

71

(KHN),

and wear resistance were considerably improved over thatfor the PFUMA/BIS-MPTS composite. In addition, the environmentalresistance of the PFMA composite was much better than that formore conventional dental composites containing rigid resins, e.g.,

BIS-GMA. The steady- state wear approximated that observed for a

microfilled composite. The results of this study were presentedbefore the lADR Conference in March 1987 [6]

.

The potential for improving the properties of the PFMA system wasrealized by employing a dual-cured (photochemical/chemical)system. The system currently under investigation is similar to

the PFMA system mentioned above, except the current one is a

powder- liquid system with siianized fine fused quartz containingbenzoyl peroxide for chemical activation. (See Section I-F)

.

Twoadvantages are achieved from the dual -cured system: (1) thedegree of cure is higher than that for the corresponding visible-light activated or chemical system, and (2) the depth of curegradient obtained from solely visible- light activation is

essentially eliminated (See Section I-F). The preliminary resultsrevealed that the KHN of the dual -cured neat resin was about fourtimes as large as that for either the corresponding visible- lightactivated or chemically cured resin. The KHN of the dual-curedcomposite, 55 kgf/mm^

,was larger than that observed for any of

our experimental flexible-resin-based composites. The KHN fellonly 27% after storing in ethanol for one week which comparesfavorably with 70% for a BIS-GMA based composite we have tested

[2] . The wear resistance for the water- stored specimens fellbetween that characteristic of a conventional composite and a

microfill, but marginally increased for the ethanol- storedspecimens, which may be an indication of mild corrosion of thefiller by water.

The inorganic reinforcing filler was evaluated by preconditioningcommercial dental composite specimens in weak intraoral acidswhich may damage the filler at the interface. Compositesemploying modified glasses with alkaline -earth elements to obtainradiopacity were more sensitive than the pure-silica-reinforcedcomposites. It was determined that corrosion plays an importantrole in the degradation of composite restorations as evident fromthe leaching experiments of Soderholm [7,8] and the wear andhardness measurements made at NBS [9]. The reason for includingPhase V (apatite fillers) was to determine if present radiopaquefillers could be replaced by apatite fillers combined with certainalkaline -earth elements to achieve radiopacity. It is expectedthat the apatite fillers will be less sensitive to corrosion fromintraoral fluids [10].

Since glass- ionomer cements show good bonding properties to dentaltissue [11], it was decided to include the evaluation of theirwear behavior in our studies. The cure process of the glass

-

ionomer cements does not seem to be as viscosity limited as with

72

the resin-based composites. Accordingly, higher degrees of curecan be reached at the in situ temperature with the former.

With the three commercial glass- ionomer materials studied, thewear resistance of the water-preconditioned specimens was goodexcept that the incidence of catastrophic failure from brittlefracture during wear was frequent [12]. Specimens preconditionedin dilute lactic acid revealed considerable chemical dissolutionas apparent from electron micrographs and accelerated wear. Morerecent work [13] on a commercial silver- sintered glass -ionomercement showed that, although the wear resistance improvedconsiderably, the susceptibility to brittle fracture and chemicaldissolution in lactic acid were still serious problems. Theseproblems stressed the need for appropriate modifications in thecomposition and structure of this class of materials for enhanceddurability.

The so-called hybrid cement-composites (HCC) formulated at NBSwere designed to overcome the deficiencies of glass - ionomercements, namely, susceptibility to brittle fracture anddissolution in intraoral acids. The experimental system (HCC)

contained a water-soluble monomer, 2 -hydroxyethyl methacrylate(HEMA)

,in the liquid (to be mixed with the powder) with

appropriate catalysts added to a conventional glass -ionomer cementpowder (For further details, see Part I, Section F) . Aftermixing, the two reactions (polymerization of HEMA and hardening ofthe glass- ionomer cement) occur simultaneously producing a "rubbertoughened" glass- ionomer cement - compos ite

,which is more flexible

than the conventional glass -ionomer cements. The experimentalresults on the HCC specimens showed that the wear rate wasslightly larger than that for the conventional glass - ionomercement, but brittle fracture failures were absent. In addition,unlike the conventional glass- ionomer cements, the hardness wasunchanged, and the wear was not enhanced from preconditioning in

organic solvents and weak intraoral acids [5]

.

Another specific source of damage to dental composites is from the

application of topical fluoride agents which may eventually reachrestorations even though not applied to them directly. In thisconnection, collaborative efforts were started with the Universityof Maryland Dental School at Baltimore to determine the effects ofvarious topical fluorides on degradation and wear of compositeresins

.

PROGRESS REPORT

Phase I. Recent Commercial Composites Which Appear Promising forPosterior Occlusal Application, (not active this period)

No experimental activity in Phase I was undertaken duringthis reporting period. Our past work on commercialcomposites provided a sufficient basis to initiate researchon NBS experimental composites. At a later time we will very

73

likely conduct some evaluations of other selected commercialposterior composites.

Phase II. Experimental Composites with Flexible Polymers

The results of the performance evaluation of the dual -cured,flexible-resin composite described earlier were presentedbefore the lADR Conference in March 1988 [14]. At the samemeeting, a paper given by Ellison [15] described results fromclinical trials involving restorations of compositescontaining PFMA resin. The investigators observed moredeterioration of anatomic form and marginal discolorationover one year than what is normally observed for BIS-GMAbased resins over the same period. Although their specimenswere prepared differently from ours, we believe that theprincipal limitation on the performance of this kind ofrestorative results from the low degree of cure. Weoriginally thought that the flexible resins would reach a

higher degree of cure because of their flexibility; however,their affinity to oxygen, which competes with the cureprocess, seems to offset the advantage of flexibility. Infuture studies employing fluorinated resins, we believe thateither we have to find a way to keep oxygen (air) out duringprocessing or incorporate a scavenger to absorb oxygenbefore, and during, polymerization.

Phase III. Glass -ionomer Cements: Conventional, Metal -Filled,and Modified Experimental.

No additional wear or environmental -resistance studies onglass - ionomer cements have been undertaken during thisreporting period. We believe that we have met our objectiveswith the hybrid cement-composite and are now considering thepossibility of implementing some clinical tests. A separatestudy of margin leakage of this material employing the

sandwich method [16] with the silver nitrate stainingtechnique [17] revealed favorable results. A manuscriptdescribing the wear and environmental resistance studies is

essentially completed and will be submitted to WERB. Apatent application on the hybrid cement-composite has beensubmitted.

Phase IV. Susceptibility of Commercial Dental Composites to

Topical Fluoride Gels.

The work on the effect of topical fluoride treatments oncommercial dental composites has been continued during this

reporting period in collaboration with the University of

Maryland Dental School. The procedure on a commercial,radiopaque, visible- light-activated composite involvedtreatments in distilled water (control), 0.5% acidulatedphosphate fluoride (APF)

,1.23% APF, 0.4% SnF

2 ,and 1.1% NaF.

74

Ten treatments were done for each specimen. Each treatmentcomprised a six-minute innnersion in the topical fluorideagent and water as a control. The mass of each specimen wasdetermined by weighing to within 10 fig before and after eachtreatment. The wear was measured after the final treatmentand compared with that for the control.

With the radiopaque composite (Fulfil) ,which employs a

hybrid, barium-modified glass filler, the greatest weightloss was observed using both APF treatments. Significant,but smaller, losses were observed using SnF2 and NaF. SEMobservations on the APF treated specimens revealed a porousstructure which is attributed to the leaching out of the

filler. SEM observations on the worn surfaces showed a

smoothing out which resulted in a polymer- rich layer. Thislayer apparently reduced the extent of abrasion, thus thewear rates, in some cases, in the early regions of wear.

More recently, studies have been made on the visible- light

-

activated microfilled composite (Silux) which contains a

quartz filler. In this case SnF2 treatments produced thelargest weight loss. With the other measurements the dataare not sufficient as yet to make any judgement. Microdefectanalyses employing a silver staining technique [17] have beenstarted on both composites.

The early phases of this work were presented at the lADRConference in March 1988 [18]. An abstract covering the

later results will be submitted for the 1989 lADR Conference.

Phase V. Experimental Studies on Experimental CompositesEmploying Apatite Reinforcing Fillers, (not active this period)

This phase has not been initiated as yet. As stated earlier,radiopaque composite fillers employing alkaline earth-modified glasses are sensitive to corrosion from the

intraoral environment during wear. The conjecture is thatcorresponding apatites, e.g. barium, will satisfy theradiopacity requirements and be essentially insensitive to

corrosion, which is enhanced by stress (stress corrosion)

.

Some work has been initiated, however, on the preparation of

apatite and phosphate fillers in collaboration with the

American Dental Association at NBS . If these appear to bepromising, some wear and environmental resistance studies oncorresponding composites may be done during the nextreporting period.

75

B . Assessment of Wear of Human EnamaL Against Dental RestorativeGoimterfaces

Background

Of increasing concern is the extent of wear on human enamel causedby occlusion with opposing restorations [19]. In our in vitrowear studies on commercial composites we found that the

conventional composites with large rough particles, although notintended for occlusal applications, produced the most wear on the

wear pins. It was encouraging to note, however, that the

posterior microfills produced the least amount of wear among the

composites tested. The wear on enamel from opposing compositerestorations has been determined to depend upon the hardness androughness (filler partial size and shape) of the composite [19].

In this connection an acceptance standard has been proposed [20]

with respect to surface roughness, which is an indication ofincreased concern of this problem with respect to composites andother restoratives. In our work we are in the course of

evaluating enamel wear resulting from commercial dental alloys andceramics

.

Phase I. Assessment of Wear of Human Enamel Against a

Conventional Porcelain and a new Castable -Ceramic Counterface.

In collaboration with the Naval Dental School wearmeasurements on human tooth enamel against rotatingcounterfaces of a conventional porcelain and a new castableceramic have been made. Contrary to what was expected, the

castable ceramic (Dicor) glazed with shading porcelainaccording to the manufacturer's specifications produced morewear on the conical enamel pins than the ungiazed ceramic.There was no significant difference between the enamel wearresulting from the unglazed ceramic and the porcelain.Accordingly, the conclusion from this study is that the

shading should not be used in areas that will articulateagainst opposing teeth.

The results of this study were presented at the lADRConference in March 1988 [21]. A manuscript has cleared WEREand has been submitted to the Journal of ProstheticDentistry.

Phase II. Wear of Hydroxyapatite Against Dental-AlloyCounterfaces, (new task)

Recently, wear studies were made involving the wear of

synthetic hydroxyapatite (to simulate enamel) discs resultingfrom wear pins made of certain commercial dental alloys and

CP titanium. The wear of the hydroxyapatite did not

necessarily increase with increasing pin hardness, as might

76

be expected. In fact, some of the very hard alloys tended to

polish the counterfaces producing very low wear rates. Insome cases two distinct types of wear were observed (1) a

surface pitting on the hydroxyapatite, which produced a

constant wear rate on both pin and disc and (2) a "glazing"of the hydroxyapatite which corresponded to an essentiallyzero wear rate. These preliminary results suggest that thenature of the hydroxyapatite surface may be a dominant factorin determining the wear rate and, in some cases, be moresignificant than the type of alloy counterface.

An abstract covering these preliminary results has beensubmitted for the 1989 lADR Conference.

C. Glass-Transition Temperature ) of Matrix Polymers (not activethis period)

Background

The glass -transition temperature, T^,

in particular as a functionof cure temperature, is an important parameter to facilitate theevaluation of the performance of resin-based composites. Withfree-radical activated systems, the degree of cure is limited by a

critical viscosity at which the molecular motion effectivelyceases, thus prohibiting further proximation of reactive species,which essentially terminates the curing process [22]

.

Since Tg is

a manifestation of an isoviscous state, the viscosity depends uponTg

,which, in turn, depends upon the cure temperature. If the

cure temperature is increased sufficiently, complete conversionmay be obtained (at least in principle) for which thecorresponding Tg is designated Tg . For the flexible polymers,the value of Tg is low in comparison to that for a rigid polymer.Thus for the former, Tg at the iji situ cure temperature and Tg

will be closer together which is tantamount to a higher degree o?

cure. On the other hand, although a value of Tg very close to

the in situ cure temperature will produce a high degree of cure,

the matrix will be too compliant mechanically to be useful as a

restorative material.

The values of Tg as a function of cure temperature have beendetermined using a Weisenberg rheogoniometer . This instrioment

employs a forced vibration (mechanical) method and measures a

complex modulus in shear comprising a component of stress in phase(elastic modulus)

,and one 90® out of phase with the strain with

sinusoidal time dependence. The 90® component, called the loss

modulus, goes through a maximum with temperature. The temperaturecorresponding to the maximum loss tangent (loss modulus/elasticmodulus) may be arbitrarily defined as Tg which, of course,

depends upon the frequency.

The values of the in phase component, called the storage or

elastic modulus, and its temperature dependence are also important

77

in regard to assessing the performance of these materials. Thesevalues describe the rigidity of the material from which thetemperature at which the restoratives break down under stress maybe determined.

Using the Weisenberg rheogoniometer,

the cure temperaturedependence of has been determined [23] for pure BIS-GMA and70/30 BIS-GMA/TEGDMA, both of which, especially the former, arerigid-pol3nmer systems and, thus, have large values of Tg^.

D. Wear Instrumentation (New Task)

Background

The in vitro wear measurements mentioned throughout this chapterwere made using a pin on disc apparatus described in Reference[24] . The original version was designed and assembled at theUniversity of Indiana [25]

.

A second version was constructed atthe National Bureau of Standards and essentially completed in1976. This version was automatic, being programmed on a tape, butdid not involve a central processing unit (CPU) . The mechanicalportion of the apparatus (involving unique componentsmanufactured at NBS) was reinterfaced, and a programmableinstrumentation controller with a CPU was incorporated [24]. Thismodification resulted in a more convenient, flexible, and reliablesystem, which has been used up to the present time. A largeportion of our activity during this reporting period was dedicatedto the design, acquisition, and construction of components to

obtain the present version of a pin and disc wear apparatus forwhich the salient features are described below.

Phase I. Methodology for Improved Wear Apparatus

As a consequence of increased demand and new methodology in

dental wear measurements, a new pin on disc wear apparatushas been designed and is being assembled. The new versionhas independent stepping motors as drives for the threerotating specimen discs and other mechanical controls. Thesemotors are fully programmable with respect to angle, rate,

and function, e.g., constant speed, sine function, ramps, or

steps. They also eliminate the need for the positioningscanners used in the present version.

As with the precursor, track-depth measurements using linearvariable differential transformers (LVDT) may be taken at ten

fixed positions around the track circumference to obtain a

reliable average; however, by virtue of the stepping motors,

this number also will be programmable.

A distinct advantage over the previous system is that the

forthcoming one will allow wear testing under arbitrarilyselected media.

78

The computer, which controls the apparatus, has thecapability to do calculations for data processing includingstatistics, and thus will not require manual transfer of thevast amount of data acquired during each wear run to a

different device.

Phase II. Selection, Acquisition, and Assembly of the ElectronicComponents

.

Most of the commercially available electronic components forthe new wear apparatus have been acquired and tested duringthe last reporting period. These include the computer(programmable in BASIC), hard and soft disc drives, a

printer, and a data acquisition control unit, which comprisesan integrating voltmeter, a 20-channel relay multiplexer withthermocouple compensation, a 16 -channel general purposeswitch, an 8-channel power controller, a 3-channel counter,and a stepping motor controller. The stepping motors, theirpower supply and control circuits, encoders, and LVDT's andtheir conditioning circuits, pulse- interrupt circuit, andGPIB cables have been acquired, or designed and constructed,during this period. The stepping motor drivers are presentlyunder construction.

Phase III. Completion of the Apparatus, Including Design andAssembly of the Mainframe.

The mainframe, which is the mechanical portion of theapparatus including the drive motors and rotating discassembly, the pin assembly, wear measurements assembly, andcam lifting assembles for the pin and measurement devices is

about 80% complete at the time of this writing. Theremaining work on the entire apparatus will involvecompletion of the mainframe, electrical interconnection ofthe components, and computer programming, which are intendedto be finished during the next reporting period.

References

[1] Wu, W. and McKinney, J.E. Influence of chemicals on wear ofdental composites. J. Dent. Res. ^ 1180-1183 (1982).

[2] McKinney, J.E. and Wu, W. Chemical softening and wear of dentalcomposites. J. Dent. Res. ^ 1326-1331 (1985)

[3] McKinney, J.E. and Wu, W. Effect of degree of cure on hardnessand wear on three commercial dental composites. AADR Abstr. ^285, Abstr. 1047 (1983).

[4] Wu, W. Degree of cure and wear resistance of dental composites.lADR Abstr. ^ 671, Abstr. 192 (1983).

79

[5] McKinney, J.E. and Antonucci, J.M. Wear and microhardness of twotypes of dental composites. lADR Abstr. ^ 848, Abstr. 1101

(1986)

.

[6] McKinney, J.E., Antonucci, J.M., and Venz,

S. In vitroperformance of an experimental composite containing a flexiblefluorinated-polymer matrix. lADR Abstr. 6̂ 128, Abstr. 175

(1987)

.

[7] Soderhoim, K.J.M. Degradation of glass filler in experimentalcomposites. J. Dent. Res. ^ 1867-1875 (1981).

[8] Soderhoim, K.J.M. Leaking of fillers in dental composites, ibid62 126-130 (1983) .

[9] McKinney, J.E. Influence of acids of wear of compositerestoratives. lADR Abstr. £3 199, Abstr. 256 (1984).

[10] Rawls, H.R.,

Legros,

R.Z., and Zimmerman, B.F. A radiopaquecomposite restorative using an apatite filler. lADR Abstr. No. 6̂

209 Abstr. 307 (1985).

[11] Hotz, P.,McLean, J.W., Seed, I., and Wilson, A.D. The bonding of

glass ionomer cements to metal and tooth substrates. Br. Dent. J.

143 41-47 (1977)

.

[12] McKinney, J.E., Antonucci, J.M., and Rupp, N.W. Wear andmicrohardness of glass- ionomer cements. J. Dent. Res. ^ 1134-

1139 (1987).

[13] McKinney, J.E., Antonucci, J.M., and Rupp, N.W. Wear andmicrohardness of a silver- sintered glass - ionomer cement. J. Dent.

Res. 67, 831-835 (1988)

.

[14] McKinney, J.E., Antonucci, J.M., and Venz, S. In vitro performanceof a dual-cured experimental composite containing a hydrophobic,flexible polymer matrix. lADR Abstr. 261, Abstr 1188 (1988).

[15] Moffa, J.P., and Ellison, J.A. Clinical evaluation of anexperimental hydrophobic and a proprietary resin. lADR Abstr. 67 .

120, Abstr. 59 (1988).

[16] McLean, J.W., Powis,

D.R.,

Prosser, H.J., and Wilson, A.D. The

use of glass- ionomer cements in bonding composite resins to

dentine. Br. Dent. J. 158 410-414 (1985).

[17] Wu, W. and Cobb, E. A silver staining technique for investigatingwear of restorative dental composites. J. Biomed. Mater. Res. 1_5

343-348 (1981).

80

[18] Kula, K.S., McKinney, J.E., Kula, T.J., and Thompson, V.P.Effects of topical fluorides on weight and wear of a dentalcomposite. lADR Abstr. ^1_, 381, Abstr. 2145 (1988).

[19] Lambrechts, P.,

Braem, M.,

and VanHerle, G. Evaluation ofclinical performance for posterior composite resins and dentaladhesives. Operat. Dent. 12

,53-78 (1987).

[20] Lambrechts, P. Braem, M.,and VanHerle, G. Roughness of enamel as

additional acceptance standard for posterior composites. lADRAbstr. 67, 262, Abstr. 1192 (1988).

[21] Palmer, D.S., Barco, G.B.

,

Pel leu, G.B.,

Jr., and McKinney, J.E.

Wear of human enamel against Dicor. lADR Abstr. 381, Abstr.2144.

[22] Wu, W. and Fanconi, B.M. Post curing of dental restorative resin.Poijnner Engineering and Science 23 704-707, (1983).

[23] Wu, W.,

and Yu, C.C. Glass transition temperature of dentalcomposite resins. lADR Abstr. ^ 229, Abstr. 487 (1985).

[24] McKinney, J.E. Apparatus for measuring wear of dental composites.Wear 76 337-347 (1982)

.

[25] Powell, J.M., Phillips, R.W.

,

and Norman, R.D. in vitro wearresponse of composite resin, amalgam, and enamel. J. Dent. Res.

^ 1183-1185 (1975).

Publications

McKinney, J.E., Antonucci, J.M., and Rupp, N.W. Wear and microhardnessof a silver-sintered glass-ionomer cement. J. Dent. Res. 831-835(1988)

.

Palmer, D.S., Barco, M.T.

,

Pelleu, G.B., Jr., and McKinney, J.E. Wearof Human Enamel against a castable-ceramic restorative. Submitted to

journal, J. Prosth. Dent.

Manuscripts Under Review or Accented for Publication

Wu, W.L.

,

McKinney, J.E. Glass transition temperature in dentalcomposite resins. J. Dent. Res., in press.

Contributed Talks

McKinney, J.E., Antonucci, J.M., and Venz,

S. In Vitro performance of a

dual-cured experimental composite containing a hydrophobic, flexiblepolymer matrix. J. Dent. Res. 67:261 (1988), Abstr. No. 1188.

81

Palmer, D.S., Barco, M.T.

,

Pelleu, G.B.,Jr., and McKinney, J.E. Wear

of human enamel against Dicor. J. Dent. Res. 67: 38 (1988), Abstr. No.

2144.

Kula, K.S., McKinney, J.E., Kuia, T.J. and Thompson, V.P.topical fluorides on weight and wear of a dental composite.Res. 67:381 (1988), Abstr. No. 2145.

Effects ofJ. Dent.

82

Ill Dental Alloys, Ceramics and Metrology

Overview

Wide fluctuations in the costs of dental restorative preciousmetal alloys have ensued over the past ten years. At the sametime new materials for fabrication of non-metallic restorationshave been introduced as have alloys of various kinds for castmetal restorations. The new materials can vary considerably in

their fabrication characteristics and clinical performance. Morereliable methods than now exist are needed to evaluate propertiesand develop methods to aid in prediction of clinical performance.For porcelain- fused- to metal alloys the prime factors includethermal- stress compatibility, porcelain-metal system strength,alloy castability and the attendant capability of producing goodfitting castings. Metrology plays an important role as thesignificance and relevance of materials properties needs to bemore fully addressed in terms of relationships to clinicalperformance, reliability and predictability. Any measurementtechnique or processes that can affect material properties orinterpretation of clinical diagnoses rightfully belongs in thiscategory.

A. Porcelain-Alloy Compatibility ('Thermo-mechanical Stress’)

PROGRESS REPORT

Overview

Residual stress from the porcelain firing cycle is considered to

be a major factor leading to failure of porcelain fused- to -metalrestorations. This study was undertaken to clarify the parameterswhich are most important in developing residual stress. These caninclude the thermo-plastic properties of the porcelain, thermalconductivities, temperature dependent elastic moduli, glasstransition temperatures (porcelain) and coefficients of thermalexpansion of porcelains and alloys.

Accomplishments

The objective of this part of the investigation was originally to

explore the potential of using the gap of a porcelain veneeredsplit metal ring as a means for determining the thermal stresscompatibility between porcelain and metal [1,2]. As this projecthas progressed that objective remains in view; however, the planhas been expanded to explore the use of reliability analysis for

prediction of fast- fracture compatibility. This will utilizefinite element modeling (FEM)

,determination of the Weibuli

fracture stress distribution and integration of the two into

reliability analysis.

83

Phase I. Complete Development of the Required Finite ElementModel (FEM)

This phase is completed, (see NBSIR 88-3782)

Phase II. Calculated effects of variables included in Phase I onstress distribution.

This phase is completed (see NBSIR 88-3782)

Phase III. Proceed with the Input of Metal Properties for Part ofthe FEM and Complete the Input of Material Parameters.

This phase is in progress. A manuscript has been preparedfor publication.

Phase IV. Devise a Simple Analytical Representation of Stress

-

Strain for Porcelain-Metal Systems, if Possible for a

Split Ring.

This phase will follow Phases I-III.

Phase V. Compare the Results of FEM vs. the Theory Developed byScherer [3] for a Simpler Porcelain-Metal System.

Phase VI. Extend the Results of FEM and Those from Part B

(Following) on Porcelain-Alloy Compatibility to

Reliability Analysis.

This is a new phase. The possibility exists that reliabilitywill be more meaningful than compatibility.

B. Porcelain-Alloy Compatibility: Strength of Porcelain-MetalSystems

Overview

The development of a porcelain-fused- to-metal (PFM) beam for theevaluation of PFM system strength under tension in four pointbending has provided a quantitative means of assessing the effectsof various manipulations on the overall PFM system strength.

The manipulations to be evaluated are: 1) the effect of reglazingon a dental porcelain 2) the effect of adding alumina to a

porcelain composition (which alters the residual stress in the PFMsystem) and 3) the effect of submersion of the specimen in waterfor one week on the overall PFM system strength.

PROGRESS REPORT

Specimen preparation has been completed for the first 2 items to

be considered.

84

Phase I. The Determination of the Effect of Porcelain Reglazingon System Strength.

This section has been completed (see NBSIR 88-3782, 1988)

Phase II. Evaluation of System Strength as Affected by Flaws.

The work on the reglazing of fractured porcelains with fourpoint bending of a PFM beam has been completed and presentedat the lADR general session in March 1988 [4]

.

The modified porcelain (filled with alumina) was prepared andthe coefficient of thermal expansion was determined as 5.5 x10“®/“C. A total of six specimens were made for thedetermination of the PFM system strength using the modifiedporcelain; however, ail specimens showed spontaneousfractures upon cooling from the glazing cycle. Acontinuation of this work is being considered; however, at

present it appears of lesser importance relative to someother projects reported on here, such as: plasmasterilization, the use of Weibull statistics in analysis ofthe properties of materials and the effects of environment onthe strength parameters of dental composites.

Phase III. The effect of Submerging the Samples in H2 O for OneWeek Prior to Fracture.

This phase will follow Phase II

Phase IV. The Effect of "Rounding the Corners" of the Specimen atthe Side of the Specimen.

Preliminary results show that the extent to which corners arerounded does not appear to affect the PFM strength, but moredata are needed.

There may be a difference between results with roundedcorners and right angle sharp corners; the results areinconclusive at this stage due to very limited data.

C . Castabilitv (filling of a Mold with Cast Dental Alloy)

PROGRESS REPORT

Overview

Because of the many dental alloys appearing on the market thereexists the need for an uncomplicated and expedient method ofevaluating casting behavior and for determining the most favorableconditions for their performance. Such a method can be useful in

85

the selection of new materials, in dental laboratory andmanufacturing process control, and in the design of new alloys.

Numerous methods have been proposed in the past for determiningvarious aspects of the casting of dental alloys. These have beendiscussed in NBSIR 86-3320.

A primary effort at NIST has been the development of a method forevaluating the ability to cast an alloy to fill a mold underprescribed casting conditions. For this purpose a methodemploying a polyester-grid mesh pattern has been chosen. (seeNBSIR 87-3539, 1987 for more details)

Accomplishments

This study has now been completed. A manuscript entitled "MeshMonitor Casting of Ni-Cr Alloys; Element Effects" has beenaccepted for publication in the Journal of Dental Materials:

In summary, the results of this investigation have shown that:

1)

Mesh monitors are viable for assessing the effects ofcasting variables on the production of complete castings.

2)

The casting of Ni-Cr alloys can be described by a castingequation [5]

:

t= a + b Eqn. 1

where T^ is the alloy superheat temperature, T^ = T^ -Tg

,

Tq = alloy casting temperature

Tg = alloy solidus temperature

Tf^ = mold temperature

and

CV t in2/3 + I

2/3 + (1-C^)i'2J

where C.^, is the castability value, defined as the fraction ofcompletely cast grid segments.

3)

Changes in investment batch affect only the term "a" in

eqn 1, changes in investment brand can affect both "a" and"b" .

4)

By solution of simultaneous equations, the effects of

individual elements on a castability value can be determined,(see NBSIR 87-3539, 1987)

86

D . Castabilitv CAccuracv of Fit of Dental Castings') .

Accomplishments

The experiment on the thermal expansion of a phosphate bondedinvestment has been performed using a total of 10 specimens.The findings indicate a change in strain which reproduciblycoincides with the onset of the phase change in the filler ofthe investment. The observed expansion, even thoughreproducible both on heating and cooling, is of suchmagnitude (20% net strain) that it most likely is anelectrical phenomenon that is temperature dependent.

The strain gauges, as they are currently available for hightemperature investigations, are not suitable for embedding incasting investments.

E. Solderabilitv

This project has been discontinued as explained in a previousreport NBSIR-87-3539

,1987.

F . Metrology and Analysis: Measurements for Characterization ofDental Materials

All current work related to this section is reported insection H which deals with relationships between metrologyand clinical performance.

G . Metrology and Analysis: Occlusal Force Indicator.

No actual development of the occlusal force indicator tookplace using the piezo-electric polymer polyvinyl idenefluoride (PVDF)

.

During May 1988, W. de Rijk attended a seminar by the

research staff of Tekscan Inc., the manufacturer of the T-

Scan system for occlusal force measurement. The T-Scan is a

system which uses a thin (60 micrometers) wafer as a

transducer and has a PC -AT type computer for data gatheringand processing. The instrument has only very recently beenintroduced and is conceptually similar to the systemsuggested in our initial proposal. The transducer in this

system is based on the change in electrical resistance in a

carbon filled polymer. The signal processing unit as made bythe company is more than adequate, but the clinicaleffectiveness of the transducer could be vastly improved witha bite wafer that is thinner and that has less rigidity. Thecurrent assessment is that if a PVDF transducer of less than10 micrometers thickness and low rigidity could be made the

T-Scan system could be significantly improved.

87

H. Metrology and Clinical Performance

Phase 1. Development of Algorithms and Software for theReduction of Data, Using the Weibuil Distribution.

The software used for the processing of fracture datahas been improved such that now the data, maximumlikelihood estimates and calculated cumulativeprobability curves can be exported to commercialgraphics routines.

The Weibuil distribution can be used to predict thecharacteristic lifetime of a given population even if

only a few early failures have occurred. The processassumes a constant hazard rate and termination orcensoring of the experiment at or near the last reportedfailure. This method has been applied to the bonding offixed prostheses. The results have been submitted forpublication. The shortcoming of this method is that allprostheses must be placed in service at the same time.

A computer program was written for the determination ofthe maximum likelihood estimators for the shape andscale parameters in time- to-failure experiments withvarious runout times (i.e. having a variable servicetime without failure at the time of the experimenttermination) . The calibration and verification of thisprogram is as yet to be performed.

Phase II Analysis of Resin-Metal Bond Tests

This phase was reported on in NBSIR 88-3782.

Phase III Analysis of Tensile Data from Composites Exposedto Food Simulating Fluids,

In light of the previous findings [6] and the hypothesisput forth that the weighting factor for a mixed Weibuildistribution, alpha, could be immersion- time dependent,an experiment was undertaken to determine the effect ofinimersion time on the Weibuil parameters for a

commercial dental composite. The material for whichprevious data were obtained is no longer available.

A different visible- light- initiated composite was usedto determine if again a mixed Weibuil distribution was

observed after the composite had been exposed to a

chemically active environment. In this study onlyethanol was used as the food simulating liquid; however,the immersion time was taken to be the independentvariable. The specific times of exposure were: 3 days,

3 weeks, 3 months and 6 months in 70% ethanol at 37 °C.

88

The diametral tensile strength was determined at a headspeed of 10 mm/min. The preliminary results are shownin fig 1. The strength as determined by a Newton-Raphson iteration of the maximum likelihood equation is

found in the table below.

S,(MPa)45.146.450.638.632.7

TIME1 day3 days3 wks3 mos6 mos

m16.317.2

14.110.86.4

The preliminary evaluation shows that no obviousevidence exists for a mixed Weibuli distribution for

this composite even though the m value changes from aninitial value of 17.2 to a final value of 6.4, which arestatistically significantly different at a confidenceinterval of 90%, using the tables by Thoman, Bain andAntie [ 7 ]

.

It should be noted that the composite under evaluationdiffers from that used previously in that the previouscomposite was expected to be more hydrophilic than the

current one (based on manufacturer's information).

Metrology and Analysis; Stress in Dental Composites Bondedto Teeth

A mechanical engineer. Dr. Martin Chiang, has been added to

the staff. Part of his responsibilities will includenumerical analysis of tooth-composite systems. At the timeof this writing, plans are being formulated for attackingthis formidable problem.

The results from the study are expected to guide:

a) development of new cavity preparations.

b) clinical studies.

c) development of new materials.

d) development of restorative placement techniques.

Metrology: Sterilization of Dental Instruments

PROGRESS REPORT

Accomplishments

Phase I. Development of Plasmas.

It was found that even with a new vacuum chamber thatthe heating of the chamber walls was a problem.Experiments using prolonged exposure to microwaveradiation with and without the concomitant vacuum showedthat the heating is not due to the absorbtion of the

electromagnetic radiation by the chamber, but onlyoccurs when an active plasma is present. A glassdesiccator proved to be an adequate vacuum chamber thatpermits exposures of longer than 2 minutes beforereaching unacceptable temperature levels.

Phase II. Effects of Plasma on Spores.

The effect of the plasma on spores of Bacillus subtiliswas determined by submerging dental burs in a sporesolution and then exposing the burs to the plasma. Theoptimum pressure for the plasma was between 50 and 200milliTorr, and the exposure time at which consistentinactivation of the spores occurred is 30 seconds. Thespores were declared inactive if incubation of the

dental bur in a tripticase broth at 37 ®C for 7 daysshowed no macroscopic cultures. Positive controlsverified the viability of the spores used in eachexperiment. Preliminary results have been accepted forpublication.

References

[1] Whitlock, R.P., Tesk, J.A., Parry, E.E. and Widera, G.E.O. Aporcelain-veneered split metal ring for evaluation of

compatibility of dental porcelain-alloy systems. J. Dent. Res.,

59A. Abstract 682. (1980)

[2] Tesk, J.A., Whitlock, R.P., Widera, G.E.O.

,

Holmes, A., and Parry,

E.E. Consideration of some factors influencing compatibility of

dental porcelains and alloys. Part II; Porcelain/alloy stress.

Proc. 4th International Precious Metals Conference, 283-291 June1980. Pergamon Press, Canada (1981).

[3] Scherer, G.W. Viscoelastic analysis of the split ring seal. J.

Am. Ceram. Soc. 66 (2) 135-139 (1983).

[4] Buchness, G.F., de Rijk, W.G., and Tesk, J.A. The effect of

reglazing on the strength of a dental porcelain to metal system.

JDR 67 Special Issue, Abstract 1211, 264 March, 1988.

[5] Hirano,

S., Tesk, J.A., and Hinman, R.W. Casting of dental

alloys: Mold and alloy temperature effects. Dent. Mater. 3(6),

307-314, (1987).

90

[6] Penn, R.W.,

de Rijk, W.G., Zapas,

L.J. and Tesk, J.A. JDR ^Special Issue, Abstract #867 (1988).

[7] Thoman, D.R.,

Bain, L.J. and Antle, C.E. Inferences on theparameters of the Weibull distribution. Technometrics IX (3) 445-

460 Aug. 1969.

Publications

de Rijk, W.G.,

Tesk, J.A., Penn, R.W. The nonnormal distribution offailure stresses in a porcelain to metal systems evaluation. Proc . 6thSouthern Biomedical Engineering Conf

. ,Oct. 1987.

de Rijk, W.G.,

Penn, R.W.,

Tesk, J.A. and Zapas, L.J. Dentalcomposites: strength properties via Weibull statistics. Proceedings ofThird World Biomaterials Congress, April 21-23, 1988, Kyoto, Japan, p.

251.

de Rijk, W.G., Tesk, J.A., Penn, R.W. and Marsh, J. Applications ofthe Weibull method to statistical analysis of strength parameters ofdental materials. Polym. Mat. Sci. Eng., Proceedings of the ACSDivision of Polymeric Materials, Vol. 59, 1988.

Hirano, S., Tesk, J.A., Hinman, R.,

Argentar, H.,

and Gregory, T.

Casting of dental alloys mold and alloy temperature effects. Dent.Matl. 3:308, 1987.

Singh, S., Katz, J.L., Antonucci, J.M., Penn, R.W.

,

and Tesk, J.A.

Ultrasonic measurements of the elastic properties of dental materials.Proceedings of Third World Biomaterials Congress, April 21-25, 1988,Kyoto, Japan, p. 118.

Tesk, J.A., Anusavice, K.J. Summary of conference on design of dentalprostheses. Dent. Mat. 4:49; 1988.

Tesk, J.A., Brauer, G.M., Antonucci, J.M., McKinney, J.E., Stansbury,J. W., Venz

,S., Lee, S., de Rijk, W.

,Penn, R.W.

,

Sugawara, A., Asaoka,K. Properties and Interactions of Oral Structures and RestorativeMaterials. NBSIR 88-3782, June, 1988.

Asaoka, K. and Tesk, J.A. Transient and residual stress in dentalporcelain fused- to-metal restorations as affected by the thermalexpansion coefficients of the alloys. Proceedings of Third WorldBiomaterials Congress, April 21-25, 1988, Kyoto, Japan, p. 518.

Conference/Special Meetings

Tesk, J.A., Chairman, Conference on Modern Instrumentation and AnalysisTechniques. Planned for October 6-8, 1988.

de Rijk, W.G., Co-Chairman, Conference on Modern Instrumentation andAnalysis Techniques. Planned for October 6-8, 1988.

91

Manuscripts accepted for publication or under review bv WERB

Thompson, V.P. and de Rijk, W.G. Clinical evaluation and lifetimepredictions for resin bonded prostheses in: Failure criteria for

Dental Restorations K. Anusavice (ed) Quintessence Int., Lombard, IL

(in press)

.

Tesk, J.A. High temperature dental investments. Book Chapter, in

press

.

Tesk, J.A. A Review of the paper: casting metals by Kamal Asgar, J.

Dent. Res., in press.

Tesk, J.A. Dental materials and technology research at the NationalInstitute of Standards and Technology, a model for government -privatesector cooperation. Proceedings of Materials Research SocietySymposium on Biomedical Materials and Engineering, November 30-Dec. 9,

1987, in press.

Engler, R.A.,

de Rijk, W.C.,

Tesk, J.A. and Morris, D. Multi-dimensional internal setting expansion of a phosphate -bonded castinginvestment measured with strain gauges. J. Prosthetic Dent., in press.

Invited Talks

de Rijk, W.C. The caries process and related measurements. JohnsHopkins U.

,March 28, 1988.

de Rijk, W.C. The non-normal distribution of failure data. Universityof Florida, July, 1988.

de Rijk, W.C. Calcium phosphate chemistry of tooth formation.Demineralization and remineralization fluoride mechanisms. The role ofsaliva in the caries process. A series of lectures as part of the

cariology course. University of Maryland, School of Dentistry, Sept.,

1988.

de Rijk, W.C.

,

Tesk, J.A., Penn, R.W,

,

and Marsh, J. Applications ofthe Weibull method to statistical analysis of strength parameters ofdental materials. ACS, Los Angeles, Sept. 27, 1988.

Tesk, J.A. Dental adhesives and their mechanical properties. JohnsHopkins U. March 28, 1988.

Tesk, J.A. Casting Today...? Tomorrow. Cal-Lab Croup of DentalManufacturers. Feb. 19, 1988.

Tesk, J.A. and de Rijk, W.C. Applications of Weibull method to

statistical analysis of dental materials. First Annual Symposium in

honor of Dr. I. Mura. Tokyo Medical and Dental University. April 20,

1988.

92

Tesk, J.A. Casting of metal and clinical significance of Weibullanalysis for dental composites. Tokushima University, April 28, 1988.

Tesk, J.A. Applications of the Weibull method of statistical analysisto the strength of dental materials. Northwestern University.September 30, 1988.

Contributed Talks

de Rijk, W.G.,

Penn, R.W., Tesk, J.A., and Zapas

,L.J. Dental

composites: strength properties via Weibull statistics. Third WorldBiomaterials Congress, Kyoto, Japan. April 24, 1988.

de Rijk, W.G. The non-normal distribution of porcelain- to-metalfailure data. 6th Bioengineering Conference, October, 1987.

Asaoka, K. and Tesk, J.A. Transient and residual stress in porcelain-metal strips. lADR, JDR, March, 1988.

Penn, R.W.,

de Rijk, W.G.,

Zapas, L.J. and Tesk, J.A. The strengthdistribution of chemically degradated composites. lADR, JDR, March,1988

Buchness, G.F., de Rijk, W.G. and Tesk, J.A. The effect of reglazingon the strength of a dental porcelain to metal system. lADR, JDR,

March, 1988.

Asaoka, K. and Tesk, J.A. Transient and residual stress in dentalporcelain fused- to-metal restorations as affected by the thermalexpansion coefficients of the alloys. Third World BiomaterialsCongress, Kyoto, Japan, April 25, 1988.

Singh, S., Katz, J.L., Antonucci, J.M., Penn, R.W. and Tesk, J.A.

Ultrasonic measurements of the elastic properties of dental materials.Third World Biomaterials Congress, Kyoto, Japan, April 22, 1988.

93

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BIBLIOGRAPHIC DATASHEET (See instructions)

REPORT NO.

NISTIR 89-4048 MAY 1989

4 . TITLE AND SUBTITLE

Properties and Interactions of Oral Structures and Restorative Materials

5. AUTHOR(S) J. A. Tesk, J.. M. Antonucci, G. M. Brauer, J. E. McKinney, J. W. Stansbury,

W. G. de Rijk, A. Sugawara, K. Asaoka

6. PERFORMING ORGANIZATION (If joint or other than MBS. see instructions)

NATIONAL BUREAU OF STANDARDSU.S. DEPARTMENT OF COMMERCEGAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant in formation. If document includes a significant

The research program described herein is designed to achieve a number of

objectives Leading to improved dental restorative materials, techniques and

applications of dental materials science for improved dental health care in

general. Some of the research in dental composites is directed toward

developing generic polymer science potentially useful for compositeapplications, e.g., durable resin matrices and stronger more durable couplingbetween fillers and resins. Improved reinforcement is sought by defining the

type, and percentages of fillers which will result in improved performance ofcomposites. Methods for reducing polymerization shrinkage and attendant stressand marginal leakage are also explored. Cements are investigated and basicformulations developed for lower solubility, higher b iocompatib i 1 i ty ,

higherstrength, greater toughness and adhesion to various substrates including enameland dentin. Analysis techniques include IR spectroscopy, chromatography, x-rayanalysis, mechanical testing, and dilatometry. Another major effort is

directed at elucidating the fundamentals involved in wear and degradation ofdental composites and restoratives. Wear and hardness measurement techniquesare pursued as well as by identification of the origins and sources of flawsleading to failure. Weibull statistical analysis is expected to provide usefulinformation for this task. In this regard an objective is to investigateimproved correlations between clinical results of wear and failure ofcomposites with laboratory test data via t ime - to - fai lure analysis. Metrologyand analysis constitutes the underlying theme of investigations into porcelain-metal systems, casting of dental alloys and the expansion of dental castinginvestments

.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

Dental engineering; dental restoratives; dental materials

13. AVAILABILITY

]U n 1 imi ted

I-Xl Official Distribution. Do Not Release to NTIS

r~n Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C,20402.

]Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OFPRINTED PAGES

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Properties and Oral Structures - NISTCONTENTS Page ABSTRACT i FY88SIGNIFICANTACCOMPLISHMENTS ii...

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Transcript of Properties and Oral Structures - NISTCONTENTS Page ABSTRACT i FY88SIGNIFICANTACCOMPLISHMENTS ii...